[mattak]

There are some good posts on implied odds (check search function). I personally use a 10x rule. Effective stacks must be 10x my opponents raise for me to call. As well, I factor in the likely hood that villian will stack off if I hit my set. It is a bit counter intuitive but IMHO you should be more inclined to call people who only 3 bet with the goods than calling light 3 betters because the tight player is more likely to stack off with their over pair. Position, of course, plays a big factor. If I have position I am more inclined to call because I may be able to play the hand to win unimproved.