Thinking on Many Levels
One of the biggest advantages that most professional players have over amateurs is the ability to think on many levels. This means thinking about what you have, what your opponent has, what your opponent thinks you have, and so on.
Thinking beyond the first two or three levels is rarely necessary in low-limit Hold'em, but as you advance in limits, it becomes necessary to think on many levels. Remember that poker is a game of information, and the better your opponents play, the more information you will need to be successful.
Psyching out your opponent takes much skill and dedication toward learning the art of bluffing. Always be thinking about your opponent's moves!