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 AQ, 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.