First of all, WiFi is fucked. 2.4 GHz is way overcrowded and just keeps getting more crowded. Lots of fucking routers now are offering increased bandwidth by using multiple channels simultaneously, etc. etc. It's one big interference fest.
The first issue I found was baby monitors. Baby monitors, like many wireless devices, are also in the 2.4 GHz band and just crap all over your wifi. Turning them off helped our signal a bit, but we were still getting constant problems.
Next issue is interference from neighbors'ses wifises. This is what inSSIDer looks like at my house :
We are *way* away from any neighbors, at least 50 feet in every direction, and we still get this amount of shit from them. Each of my cock-ass-fuck neighbors seems to have four or five wifi networks. Good job guys, way to fix your signal strength issues by just piling more shit in the spectrum.
What you can't see from the static image is that lots of the fucking neighbor wifis are not locked to a specific channel, many of them are constantly jumping around trying to find a clear channel, which just makes them crud them all up.
(I'd love to get some kind of super industrial strength wifi for my house and just crank it up to infinity and put it on every channel so that nobody for a mile around gets any wifi)
I've long had our WiFi on channel 8 because it looked like the most clear spot to be. Well, it turns out that was a classic newb mistake. Apparently it's worse to be slightly offset from a busy channel than it is to be right on it. When you're offset, you get signal leakage from the other channel that just looks like noise; being on the channel you're fighting with other people, but at least you are seing their data as real data that you can ignore. Anyway, switching our network to channel 11 fixed it.
It looks like in practice channel 6 and 11 are the only usable ones in noisy environments (eg. everywhere).
The new 802.11ac on 5 GHz should be a nice clean way to go for a few years until it too gets crudded up.