Regardless of the specific game, there are several keys to being a successful poker player.
Be conservative. Poker players, and even entire poker games, can be characterized as either tight or loose. A loose player stays in every hand until the showdown, bets and raises with mediocre hands, and generally loses more money than he makes. A tight player folds most hands before ever making a bet. He doesn't spend money throwing chips into the pot to save earlier bets. He knows a bad hand is a bad hand, and he won't play with it. Poker is a game of patience. Good players know they will lose a little money on many hands, waiting for the right hand to come along.
Know when to be aggressive. It's certainly possible to play too tight. When a good hand comes along, raise, and raise again. That's when a good poker player earns her money.
Know the odds. You don't have to be a mathematician, or even memorize long tables of statistics. Just be observant and consider how many cards of each kind are in a deck.
In a game of Texas Hold'em, if you have three of a kind, this is usually a good hand. But if there are four Diamonds on the board, chances are at least one other player has a Diamond in his hand, and that Flush would easily beat your set. Time to fold.
In a Seven-Card Stud game, imagine that you're holding four Clubs to a Flush. You can bet aggressively on drawing that fifth Club if there aren't any others showing, because that increases the odds that there's another one in the deck. If there are already five Clubs showing in other players' hands, the odds of drawing to that Flush are significantly lower.
Weigh cost against the pot. Sometimes you might have a hand you wouldn't normally play with. If you've stayed in the hand, and you're close to the river, consider how much is in the pot versus how much you have to call to stay in. If it will only cost you $5 to stay in a hand with $250 in the pot, it's worth the risk. You could lose with that hand 40 times, but if you win it once when there's a significant pot, you'll make money. A big pot can make bad odds good.
Have standards. You won't have time to consider odds and ponder all the possible outcomes of a hand at the table. Know the game you're playing, and know what kind of opening hand you should play with. If your opening draw doesn't meet your standards, fold.
Bluff rarely. A bluff works best when no one expects it. They won't expect a bluff from a conservative player, or from one with a reputation for never bluffing. You have to bluff occasionally, though. Otherwise, the other players will know you have a good hand any time you bet aggressively, and they'll fold.
Although poker is game of chance at its core, there are good poker players and there are bad poker players. A bad player will get lucky now and then, and every good player hits stretches of bad luck, but over the long term, a good player will make money at poker. In fact, there are quite a few professional poker players in the world.