Do you mean with people, or just everything in general?
For general, I've heard (but haven't tried yet) that an effective exercise is to start forcing yourself to make useful new observations every day. It doesn't have to be anything profound, just be more observant all the time. For example, next time you're walking a familiar path, notice where the big crack in the sidewalk is, notice how many steps are on the stairs in your house, notice the colour of your neighbours door, etc. Over time this should get you into the habit of noticing a lot.
For people, unfortunately I find my ability to predict behaviour greatly increased when I started being more cynical. A LOT of behaviour is easily explained if you think of people acting only in their own selfish interests.