Follow the money
After playing Poker now for years with all kind of players I found that there is one single mistake that players of all level do. They call too frequently. They call when they really should fold.
Nobody like folding hand after hand, but if your goal is to win money at the poker table, that's exactly what you will have to do. You might not have as much fun as your opponents - at least not until you walk out of the casino with a lot of chips and they are left with a pocket full of hope.
Now that we are aware of these tendencies of the other players, we can start thinking how to take advantage of these tendencies. When a player will call too often and fold too infrequently, he will not be easy to bluff. But you can get him to call with a weak hand that he wuld chick if teh choice was his. Save your bluffs for good, strong players and bet your marginal hands for value against players who call all the time. Weak opponents are the type who will call with a lesser hand then yours.
In a tournament or a no-limit cash game you might not want to play as many marginal hands. But when you do make a big hand, you can nickel and dime your adversary into calling his money away. Particularly if you are able to price him into the pot by betting just enough to seduce him ino calling with losing hand. Once you reach the turn or the river, you can put him all-in for his remaining money. Now tat the pot has grown so large, he is very likely to call himself right out of the tournament.
In a limit game, you don't have to be wary of betting the river if you bet the turn and were not raise. Unless the river card makes an obvious flush or straight for an opponent chances are good that your hand is still the strongest one in play.
Because texas hold'em is such a front-loaded game, hands that are good on the flop and the turn tend to be good on the river too.