One of the things I always do is watch the hands being played that I'm not involved in. As bets are made and then raises, etc. I ask myself what do you have to have to call this bet/raise/reraise?

For example, (you're not in this hand) it's limped around preflop and 5 people are in. Flop is K-Q-10. Action on the flop is bet pot, raise pot, reraise by a third player. I think to myself, what does the first and second player have to have to call these raises? Normally, it's A-J, of course. I think I can guess my opponents' proper play the vast majority of time.

I think this helps because you then get in the habit of thinking the same thing for yourself when you're in a hand. OK, he raised on that board what's the minimum I need to call this? I don't have that, so I'm folding even though I flopped the nut flush. I think this exercise helps a lot in improving your postflop play.