One thing I started doing a few months ago which I wasn't doing before is to really analyze the texture of the flop. This forces you to think about what sort of hands the opponent would play on that flop. Combine that with their preflop action and you can usually narrow the hands down pretty well. If the turn action seems to contradict what happened on the flop, go with the turn action (usually either the opponent was overplaying on the cheap street or slowplaying - when the bets get bigger the true hand strength tends to come out).
NeverWin had some good points about this on his site:
NeverWin's guide to heads up