I was at my first World Series of Poker in 2002, talking to a player who had made the final table the year before. He told me something I've never forgotten, and it's helped me ever since.
I had raised pre-flop with A-K and he called from the button. The flop came all small cards. I checked and he fired a pot-sized bet. I looked at him and said, "You must have a good hand." His reply caught me off guard; "It doesn't matter what cards I have if I know what cards you have."
At first I thought I might have a tell - maybe I hummed when I missed the flop, or I looked away from my chips. It was later that I realized I did have a tell, but it had nothing to do with my physical demeanor. It was the way I played my cards.
Poker is often not so much about the cards you have, but knowing the way your opponent plays. Keeping track of which hands your opponent raises with - and the position from which he raises with them - is a large part of the game.
In a live game, it is hard to remember exactly what cards your opponent has raised with over the years and, if they're good players, those hands will change from time to time. But many poker players are creatures of habit, playing the style they are most familiar with. Online, there is no excuse not to have this knowledge at your fingertips.
While playing on Full Tilt Poker, I get to write notes on players and it is a great help. I am always referencing my notes, and they will often tell me which hands an opponent has played in the past. The color-coding makes it even easier for me. I use one color to mark the players who only bet when they have a strong hand, and another color to mark the action players.
When I see a player marked with a certain color, I can safely assume that he's going to overplay his hands. This is a guy I am more willing to call with a hand that might be a little weaker, or a drawing hand after the flop. Why? Because I know that if I hit my hand, he's going to pay me off; I have implied odds to call. With another player, I'll play a little tighter because not getting paid off means my implied odds aren't there. This one bit of information has both increased my winnings and minimized my losses.
Self-awareness is an important part of any endeavor. But in poker, knowing your opponent is just as important as knowing yourself.