There are many reasons to bluff. If you consistently only play hands that win, other players start to realize this and fold when you bet. So, you have to change up your game a little. A good way to do this is to win a few hands in a row, and bet a solid equal amount each time. This way, on a hand where you have bad cards and no one is betting, you can fool everyone into thinking you have a much stronger hand. Another commonly used bluffing strategy is attempting to steal a pot. You can do this as long as you have a fairly large chip stack to support anyone who ends up calling your bluff. Along with that, when people are betting on their starting hands, you could try and steal the pot with a bad flop.
Common Bluffing Strategies
First you must try to single out the pot so it's between you and another player. Next, judge that persons hand based on how they have bet in the past compared to how they are betting now. For example, if the check then you might sense weakness, but you should think of past situations, because that player may be slow playing. This is a common trap the beginners will dig themselves into. If they wee the better, see what happens as you raise them (proportionately, of course). If that player calls your raise, they will almost always have a good hand, furthermore if they re-raise, you know you've gotten yourself into a little trouble.
Mix it Up
Try not to bluff too often, as people will quickly catch on to your playing style and begin to push you out of pots. When bluffing, you want to change your style as often as possible. For example, if you're dealt a strong hand in the hole, you can check and trap them into believing you have an average hand. This slow playing technique should draw in potential bluffers who will try and bet against you, but soon realize they made a mistake. Be conscious of how you bluff and how you play regular too, and try to mix your actions around. Confusion and uncertainty are the key elements to bluffing successfully.
The Reverse Bluff
Another good way to bluff is to pretend like your playing a different hand than the one you actually have. If you are dealt a J10, play it as though you were dealt a pocket pair, and hint some of the other players into believing your playing for a pair. This way, you can choose your bluff or non-bluff action inversely. If your straight or flush comes, you will almost fake-bluff the hand. If a high card comes (matching with your pocket cards or high cards bluff), other players will see this and realize you've hit something, even though your sitting on nothing at all.
Muck Everything
As a general rule, I always muck my cards after winning. Many players who play in a friendly environment will show their cards after winning, but then when you bluff and muck them people will know you were bluffing. The only truly safe was is to consistently muck your winning cards so that no one can catch on to your play.