I'm going to write myself my guidelines. My computer system guesses the spread that it thinks is the "true spread". I want to decide to bet based on the position of the TS and the VL (Vegas Line).
1. If the TS and VL are on opposite sides of the zero, I like betting the Vegas dog to win. I especially like it if it can mean betting an underdog on the moneyline where I get +150 or some nice odds like that.
2. If the TS is close to zero and the VL is big, eg. if the TS is like 1 and VL is like 4, I again might bet the dog to win if my judgement of the intangibles is that they have a good shot. If not I drop down to the next rule :
3. If the TS/VL are on opposite sides of the 3 or 7 with at least 2 points of difference, bet the spread. eg. if the VL is a 9 and the TS is a 6, bet the spread. Teams will often win by exactly 3 or 7, so when you can bet across the 3 or 7 you're getting good value from the spread. On the other hand if the VL is a 6.5 and the TS is a 4, probably don't bet unless the intangibles really like the bet because you're not crossing a 3 or 7.
4. To make the intangibles a bit more concrete : A) Prefer betting home dogs. B) Prefer betting teams that "need a win" vs. teams that are either coasting to the playoffs are completely out of it. C) Beware of teams whose past performance is not a good indicator of their true quality, eg. if you judge they've gotten lucky in a lot of games. D) The computer isn't aware of news or injuries so mainly just avoid games where those are big factors.
Update : this week there seems to be a huge edge betting the Ravens to win vs. Cincinatti.