I'm going to go over some of the basics which everyone should know, and also some thoughts that perhaps most of the ergonomics guys don't talk about because they aren't computer users, and some things I've learned through my shoulder injury.
Constant sitting and computer use is one of the most destructive things you can do to the body. Not only do you put the appendages in tightened positions, which can pinch nerves and cut off blood flow, you often place big pressure on the spine which can cause the vertebrae to shift, and the hours and hours of sitting with no activity cause the tendons to shorten and the muscles to atrophy. It's the atrophy of stabilizer mucles which may be the most harmful, because it means you cannot support yourself with your muscles and hold good posture, and instead you rely on your skeleton to support you, which leads to all the other injuries. It also means that any time you do something athletic you're not bearing the forces with your muscles, which leads to more injuries. People who sit do damage to their knees, hips, back, neck, shoulders, elbows and wrists !! (only your ankles are safe)
Let me start with the summary : it's good to know the basic "ergonomic" ways to sit (and good to have a bunch of different options), but that is only the first small part of the battle. The real solution is to get out of an inactive, sitting, hunched-forward, atrophied- muscle life. You need to do exercises to correct the bad habits and posture of computer users. You need to sit "actively" using your muscles and moving around. You need to change positions constantly, take breaks, stretch and rest. If all you do is sit and use computers, your body will be wrecked regardless of how well you sit.
First some review of the standard advice. Everybody by now should know the "90 degree position". Feet are on the ground, knees bent 90 degrees, sitting on your "sit bones", hips at 90 degrees, neck straight up, shoulders back, humerus straight down, forearms level. Okay, this is the "90 degree position" which is commonly advocated, but it's only sort of okay.
Basic sitting style : You want to sit on your "sit bones", not your butt. You can feel the two hard bones around the base of your butt where it meets your legs. To sit up on them, lean forward slightly and engage the ab muscles to hold your body erect. It may help to imagine that someone is holding your skull and pulling you upward by the head. Puff out your chest, engage the ab and back muscles slightly in neutral position. This is easiest to practice on a firm bench like a piano bench. Your spine should be in a slight "S", going inwards in the low back and outwards in the upper back. Okay, now you know this. You actually want a chair with a back when sitting so that it helps you keep this posture, but you need to imagine that you're sitting up with your muscles, and the chair back should provide uniform pressure across your whole back to just help you. Anyway, this is also bad.
The problem with both of these is that it's just too hard to hold them for any length of time. If you're going to be coding 8+ hours a day, you will not be able to hold these positions with your muscles and you will begin to let your weight rest on your bones and cartilege instead. These positions are very very hard on the body if not supported by muscle. BTW the very best thing you can do for your postural health is to get stronger muscles, you need a very strong abs & back and shoulders to be able to sit all day. Ironic, I know.
The head should be up and "back" and not looking down. I say "back" in quotes because really it should be a neutral position, but just about everyone has it forward, so you need to push it back from what you have be doing. The bones of the neck are in a neutral position roughly straight up when you're sitting right or perhaps very slightly angled forward. Your monitor needs to be high enough that you can look pretty much straight ahead. The ideal spot is roughly where if you look straight ahead that's about 1/4 of the way down the screen (pretty much no monitor is tall enough on its own, you have to put something under it). While I'm talking about monitors - the common dual screen setup that coders use is very bad. You should not be turning your head to either side for any signficant length of time. If you need to look to the side, you should turn your whole body. It's much better to have one large monitor than two. If you do have one large monitor, make sure your windows are centered, not left-justified, as that would cause you to be looking slightly left all the time. Bad neck position squeezes the discs in the spinal cord around the neck and shoulders, which can impinge the nerves coming out of the spine and going to the shoulders and arms. This can cause weakness, muscle spasm, numbness, constant muscle tightness, and pain. Once you get vertebra damage, it's basically impossible to fix by any means. Seriously, don't get it. Your head should be far enough from the monitor that you don't have to look very far in any direction to see the whole thing.
Shoulders should be back and down. Again, this is just the neutral position, but so many computer users are hunched forward that you really need to focus on getting the shoulders back. It's basically impossible to have them back too far, so go ahead and hold them back as much as possible. You should be retracting using the scapula muscles in the mid back (rhomboids), not hunching the shoulders up with the trapezius. The same goes roughly with getting them down - it's pretty impossible to hold them down too far for any length of time, and lots of people have them constantly hunched up, so just try to keep the shoulders down as much as possible. Note that arm rests on chairs usually get in the way of this, so you want a chair with no arm rests or removeable arm rests which you can take off. When reaching for anything - the keyboard, the mouse, etc. - the shoulders need to stay back, don't reach out with the shoulder. Basically this means that anything you reach for regularly should be within forearm distance of your torso. Elbows should be close to your ribs at all times. One way to be aware of the right position is to pay attention to how your scapulae feel against your chair-back. You should feel the flat surfaces of your scapulae flush against the chair back (when you lean back) - not the points of your scapula sticking into the chair.
Forearms should be roughly level, and wrists should be level or slightly down, and relaxed. In particular, the arm should be supported from the shoulder, not by resting the weight of the arm on the hand. Many people use these wrist pad things. Those are certainly better than resting your wrist on a hard surface, but they encourage a very bad habit of resting the arm weight on the pads. Split keyboards are nice, but the MS ones are awfully thick, which means your desk surface needs to be really low to avoid having the key surface too high. Usually this means that the desk needs to be as low as possible such that you can still get your legs under it. You should not be able to cross your legs under your desk.
The head forward shoulders hunched posture of the typical computer uses is called "kyphosis", which is a forward rounding of the upper spine. It's bad for the vertebrae as well as the function of the scapula, the shoulder muscles, and the load bearing function of the core. These disfunctions make simple activities like holding a weight over your head very dangerous. One way to feel if you're in danger is to run your hand over the back of your neck. You can feel the vertebrae. Feel near the level of the shoulders, the vertebra here is C7 and it will be a pronounced protuberance if you've had your neck too far forward for a long time. (BTW another contributor to Kyphosis is the modern obsession with pecs & abs; even people who do work out will often overtrain the front of the body leading to constant hunching forward).
Typical computer users are in a severe state of muscle atrophy. It may feel very straining just to sit up without back support, such as when sitting on a physio ball. Similarly it may feel very difficult on the upper back to hold the shoulders back. Neither of these should be difficult for someone with even a basic level of body function. An immediate course of physical therapy and stretching is warranted to correct these problems. I'm not going to go into a ton of detail right here about the best exercises and stretches, but they can be done as often as every day, and a full course would take about an hour a day. Exercises should start pretty light and involve isometric holds with each repetition, using high-rep sets, something like a 3x10 pattern. Once some function and stability and posture is acheived the exercises can be done in the more typical hyptertrophy range of higher intensity.
On equipment : buyers should procure chairs that are highly adjustable. If you can find a chair that perfectly fits your body in a 90-degree sitting position, that's fine, but for office managers you need chairs that can be adjusted to any employee. That means height adjustment, removeable arm rests, back tilt (back tilt should not tilt the seat), and adjustable lumbar support (preferrably in depth as well as position). Desks should also be height adjustable. The height of the mousing surface needs to be about 1 inch above the users waist in a 90 degree sitting position. Note that this also requires that the desk top should be very thin, and there should be no support bars under the desk top where the users knees will go. Height adjustable keyboard and mouse trays are one option, but in that case the desk top needs to be very high, and most cheap trays are really flimsy and horrible to use. Height adjustable thin-top desks are quite cheap and all office managers should procure them. The exact keyboard and mouse that a user wants is not really a big deal, they can use what they like. What is important is that they can put their hands in position on those devices while keeping the back and shoulders neutral. That can be hard to arrange, though it's easier with a track ball or a chair-mounted mouse tray.
Okay, this is a good start, but this will still wreck your body. For one thing, as mentioned before, it's just too hard to hold this position for a long time. But even aside from that it's bad. Your hips are not made to be bent 90 degrees like that for long periods, they need to be straight. The combination of hips bending and knees bent leads to severe hamstring shortening and hip tightness which is very bad and dangerous for athletic performance. A similar thing happens in the shoulders - having them down and immobile all the time leads to atrophy of the shoulder girdle. Something that most people aren't aware of is that the shoulder is not a ball and socket joint. Rather, the head of the shoulder is simply held to the glenohumeral joint through muscles and tendons. It's like it's just strapped on there with soft tissue, and when that soft tissue atrophies, you're at increased risk of dislocation, as well as soft tissue injuries like rotator cuff tears, "slap" injuries and seperations. Shortened and immobile joints also lead to nerve shortening and loss of blood flow. This "90 degree sitting position" that we've advocates is almost a fetal position with all your joints curled up and shortened and it's just horrible for you.
So, what's better? Well, not much. One common alternative that's advocated is a "kneeling chair" (sometimes called an "ergonomic chair"). These things provide pretty much no benefit, but could be used as part of position cycling (see later). Another device I have used is sitting on a "physio ball" (big blow up balls). Make sure you get a ball the right size and blow it up so you can be in a 90 degree position. This is a useful training tool to help your sitting posture, because it engages the muscles and makes you aware of posture, but you should not sit on it for more than 30 minutes or so at a time as it's very fatiguing. A simple bench or stool at the right height can serve the same purpose.
The real best thing you can do is two part : position cycling and taking breaks. You need to take a 5 minute break at least once an hour. I know this is really hard to do, but there's no substitute for it. The break should involve some simple stretching and active mobility work. Position cycling means not sitting in the same way for long, ideally using as many different positions as possible. One option would be to change positions every hour when you take your break. A better option is to just be changing positions and stretching constantly. Any time you start up a program and it's taking a second, stand up! When you compile, stretch your arms out to your sides, then up over your head. When resting or waiting for something, don't just sit there - move around. There are various free & not free programs to help force you to take a break. These can be very useful to get you in the habit because most of us won't take enough breaks if left to our own devices.
Basically you need to stop resting on your skeleton and ligaments all the time, and start using your muscles. But you don't just want to lock up your muscles and try to hold the "90 degree" position. You want to stay as relaxed and mobile as possible. You want to keep the body moving in natural ways and stretch and let the muscles move around and contract and relax. There's also really no substitute for getting plenty of exercise outside of work. If all you do is sit at a desk and then sit at home your body is going to be wrecked no matter how "well" you sit.
BTW I haven't mentioned the most imporant thing, which is using a computer less, because I presume it's basically not possible. One thing you should work on is getting away from the computer when you don't need to be at it.
As for position cycling, some of the useful positions : 1. regular 90 degree sitting, 2. sitting on a ball or a pogo-stick chair where you're "actively sitting", 3. reclining in a normal desk chair; this is actually a very good position, but you have to be careful. Recline from the hips with a straight back, not a slouch in the low back. Make sure you can still reach your keyboard and mouse near your lap, not reaching up or straining the shoulders. You may also need to be able to elevate the monitor to make it high enough that your neck can be neutral, not tilted forward relative to your torso. 4. standing up. Standing up is one of the very best work positions you can have. You will need to elevate your keyboard and monitor a lot, so you probably need a "sit to stand" desk, which is one of the best pieces of ergonomic equipment you can get.
Having no desk surface at all and having a wireless keyboard in your lap is an interesting option. The standard keyboard with numpad presents a lot of problems for mouse placement. Putting the mouse off the right side makes it too big of a reach. If you have a corner desk, the mouse can be in front of the numpad. Alternatively the mouse could be on a tray or on the chair, or it could be a trackball.
Frequent use of laptops is just horrifically bad. They do just about everything wrong to your body and really actively promote the hunched kyphotic posture. It's highly discouraged.
Let me sum up and emphasize that the solution is not any particular "ergonomic position" or any piece of equipment you can buy. It's a lifestyle. It's a mentality of listening to your body and putting your body before your work. It's about being mentally aware of how your body feels and keeping your "mind in your muscles" - feeling your abs and scapular adductors holding you erect, not just resting on your frame. It's about stretching and exercising and resting every single day. You need to start listening to your body. If you really listen, your body will tell you when you do bad things to it - it's just that you're so used to abusing it constantly that you automatically ignore it.
BTW if you want to do exercises that will be beneficial, some of the things you should focus on are strengthening the back, fighting kyphosis, strengthening the shoulders, in particular the posterior shoulder girdle such as the scapular retractors and the rotator cuff, hip mobility and hamstrength stretching movements, and in general extension and pulling movements. Rowing is actually a superb all-around full body anti-computer-use movement which does most of these things.
It may be impossible for someone who's chronically heavily using computers to really fix their neuromuscular patterns. One suggestion that might help is the next time you take a week or two vacation, try to really exercise and stretch and treat your body well during that time (do lots of swimming and rowing and yoga and good active mobility and extension work). Now when you return to work be aware of how healthy your body feels. When you sit down, keep that feeling. If the work starts to make that feeling go away - fix your work pattern.
Another addendum : if your workstation is not set up well, it doesn't matter how much good work you do away from work. It's valuable to know body-friendly positioning and desk setup, even though that is not the "solution". Basically sitting at your desk is wrecking you, and movement and strength is restoring you. If your desk has too much wrecking power, you can't beat it. You want your workstation set up to be as non-damaging as possible. It will *always* be damaging, no matter what kind of active sitting you do, but it's important to minimize how bad it is, as well as minimizing your time spent sitting.