If you have a good hand, you should be raising. For one thing, if you call and then hit a draw, it will be obvious when you change direction and start raising. Of course, there are exceptions: If there have already been two pre-flop raisers, you wouldn't want to raise a pot with Jacks.  Also, it's best to not bet your draws if  more than a couple of other players are in the pot.

Calling:  A Sign of Weakness

Especially in No-Limit games, a player who calls constantly indicates a fish saying "Show me what you've got." Calling hands too often can easily lead to trouble. If your hand isn't good enough to raise, it's probably not good enough to play.

If you call pre-flop, most good players will bet you out of a pot that you should have won. On the other hand, if you raise pre-flop, it shows strength, and many players who missed the flop will fold to your bets.

Raising: A Sign of Control

Raising a hand shows dominance, because it reveals your serious intentions. If your opponents miss the flop, you'll take no prisoners. This applies to any type of poker game, but especially to No-Limit games. In a Limit game, it's more difficult to bet people out of pots because betting is usually small and constant; it's generally easy to bluff a short-stacked player when you raise, though.

When you raise a hand, you should then usually bet the flop if there are less than three other players in the pot. If you show strength pre-flop, your opponents will play cautiously, even if they hit the flop; players rarely hit the flop enough to call your raise.

An example of successful pre-flop raising: A couple of players limp in pre-flop, even though one has a pair of 8s in his hand. With an Ace-King in the hole, you raise. One of the opponents who limped in calls your raise.The flop does you no good (J-Q-5). But you bet out, and the opponent folds, even though he had the better cards.