I think it was Harrington on Holdem where I read this. Start with the player to your left. Observe him, try to get a feel for what his range is UTG to BB. See how close you are every time he shows. When you start to see that you are getting him down, include the player to his left. Same thing. Then include the player to your right. Make this something you consciously do every hand, every game. The more you do this, the more you will see yourself improving. Keep adding players to your list as you go. As pokerstudent said, every hand, whether you are in it or not, is important. It is even easier when you are out of the hand, as you are under no pressure to make decisions, and it can be fun to try to put all the involved players on hands. And, also as mentioned above, do this religiously and you will see improvement, probably sooner than you think.