Outplay your opponents, but don't Overplay them.

This is an important concept that should be put into practice at every table you sit down at.  You can be the best player at the table by a long shot, but if you're overplaying your opponents, you will consistently lose.  The concept is very simple.  There are several different levels of players:

Level 1: These are players on tilt, or occasionally drunk players.  They don't think whatsoever, not even about their own hands.  They are overly reckless and willing to put money in the pot with any two cards no matter what they are and whats on the board.

Level 2: These are beginning players.  They only think about their hand and give no consideration to what you might be holding.

Level 3: These are slightly advanced beginners.  They think about what they have and what you might have, but the thought process goes no further then that.

Level 4: Most players fall into this catagory.  They think about what they have, what you have, and what they think you think they have.

Level 5: The more experienced players will fall into this catagory.  They think about what they have, what you have, what they think you think they have, and what you think they think you think they have.

I'm not going to go any further then that, because it just becomes a nuisance to type out, but the levels could conceivably extend infinitely.  They don't really, but it's impossible to know where they stop without playing everyone, and you'll rarely run into players above level 5 except at the really high stakes anyway.

Now, the reason it is important not to overplay your opponent is because if you are playing at level 4 you are going to make decisions based largely on what hand you think your opponent is putting you on.  If your opponent is only playing at level 2 then he is not putting you on a hand at all, and making decisions based on something he's not thinking about will lead to a lot of mistakes on your part.  Even though you are clearly the better player, there is a good chance he is going to beat you if your decisions are based on something he's not thinking about.

Of course, you still need to outplay your opponents in order to win.  In order to do that consistently you need to be thinking one level ahead of your opponent.  This becomes difficult with players you don't know because you first have to determine what level they are playing at.  There is not a lot of advice I can give on determining what level your opponents are playing at except to watch carefully.  Don't try to get tricky for the first few rounds and pay attention to how your opponents are playing.  Play relatively safe and, once again, WATCH CAREFULLY.  After a couple of rounds you should have an idea of where most of the players at the table are at.  Now is the time to switch gears and start playing a little more aggressively, focusing on playing each player one level above where they are playing.  This can change throughout the course of a game so you still need to be constantly evaluating each player at the table.  If you aren't involved in a hand it is a great time to pay careful attention to each player that is involved and reevaluate them because you have nothing else to think about.