You've probably heard the standard reason to avoid slow playing: It's dangerous, because when you slow play, you give an opponent a chance to make a bigger hand at a minimal cost. This is absolutely true. But there's another reason to play your big hands fast, and this one isn't talked about as frequently. A slow play can give an opponent a chance to get away from a hand more cheaply than he would have had you played it fast from the start. Consider the following example.
You're in late position in a No-Limit Hold 'em ring game. A player raises in early position. You look at your cards, see pocket 8s, and decide to call. The flop is absolutely perfect: Qh 8h 2d. You've hit your set and, with the Queen out there, chances are your opponent has something – maybe A-Q, maybe pocket Kings or Aces. He bets the flop.
Many players will just call in this spot, hoping to get their opponent to bet on the turn. But a raise is usually the better play. If you just call, you risk seeing a heart on the turn. I don't think you need to be especially worried about the flush beating your set. You might get your set beat by a flush draw even if you raise. However, you do need to be concerned about the effect the third heart will have on your opponent. He very well might suspect that you were on the flush draw and he'd no longer be willing to commit a lot of money to the hand, even if he has Aces.
In fact, any King, Jack, 10, 9 or a card that pairs the board is likely to give your opponent pause. If he bets on the turn and you raise, you're signaling that the turn card helped you. In effect, you're saying that you liked the flop enough to call and the turn improved your hand in some way. You're announcing that you can beat one pair.
So the flop very well may be the only time when your opponent is willing to make a stand with a single pair. If he bets the flop of Qh 8h 2d and you raise, he's likely to think that you're semi-bluffing -- raising on a flush draw. At that point, he might feel compelled to protect his hand with large re-raise or perhaps an all in. When this happens, you'll take down a monster pot.
It's OK when a flop raise doesn't get you the result you want. You might scare off someone holding pocket Jacks or Ace-King, but you wouldn't make a lot of money off these hands anyway. And, if you're up against Ah-Jh, you may lose a big pot to a flush. But that's OK, because you'll have gotten your money in with the best hand.
Of course, there are some occasions where slow playing is the best choice. If you flop quads or something like Queens full, you'll want to give an opponent a chance to make some kind of hand on the turn or river. But frequently, the best option is to play fast on the flop. It may be your only chance to win a big pot with a big hand.