Bluffing

There are two reasons to bluff, 1) to vary your play and disguise your moves (eg. so that you win bigger pots when you actually have a hand), 2) to get people to fold and win the pot. I wouldn't worry about #1 too much unless your play is getting too predictable. You should bluff to get people to fold. The key here is sensing weakness. If one player bets and other raises, you probably should not re-raise bluff; they're not both going to fold. Also, the bigger the pot, the less likely people are to fold, and people are more likely to fold to the bigger turn/river bets than the small flop bet.

Stealing from the button is a very common bluffing situation. On the flop, for example, if you are on the button (last to act) and it goes check-check-check to you, everyone has shown weakness, and you can often bet and win the pot. A bet in this situation is almost automatic at some tables because people lay down their hands too often. Sometimes a raise-bluff on the river can work very well, if the flop and turn go check-check and then someone bets the river, they're often trying to steal the pot, so a raise back at them will often make them fold. Reading players and the situation is crucial to bluffing, and it's a skill that comes over time. Don't do too much of it too early.

One crucial thing about bluffing is that it doesn't have to work very often to be profitable. As I noted before, if the pot is 20 bets and your opponent will fold 10% of the time, you should always bet.

Bluffing works best when your opponent doesn't think you're bluffing - they think you have a hand that beats them, so they have to lay it down. Sometimes bluffing even works when they know you're bluffing, just because their hand is so bad that they think you'll still win, or because they don't want to risk their stack. That applies best in no-limit hold'em (see below). Note that if you bluff a lot, people will pick it up and adjust and start calling you down.

The key to profitable bluffing is for it to work more often than you it should for how much you are betting.  For example, if you are bluffing pot-size, then it needs to work more than 50% of the time to be profitable.  More than that, it's a good play, less than that, it's a bad play.  You want your bet size to be as small as possible such that you can win the pot.  For example, once your bet is larger than 2X the pot, you're probably not going to get any more hands to fold, so all you're doing is risking more chips for no reason.

The cards you bluff with are not generally very important.  If you have a legitimate draw, especially if it's on the flop or preflop, then you may be on a semi-bluff, where you can win two ways, either by them folding or you making your draw (see the section on semi-bluffs).  If you are on a pure bluff, then you cannot beat any hand they would call with.  In that case it really doesn't matter that you have Ace-high or a low pair or something like that, your hand is basically "junk" and any hand that will call you beats you.

It's important to know that you can win a hand at any time by bluffing.  You don't need to have a good hand to win, what you need is for 1) your opponent to have a weak hand, and 2) for him to actually believe that you have a big hand.  That means your body language and betting pattern need to at least suggest the possibility of a big hand.  If you have obviously played your hand very weakly, he's not going to believe it when you make a big bet.  If your opponent shows obvious weakness, you should almost always play back at him hard, confidently, don't even give a thought to how weak your own cards are.

It's important to also remember that when you make a bluff, you are just winning the pot.  The amount you are betting is coming from your stack, you are not winning those chips - on the contrary, you are risking them!  If you can win the pot without bluffing, that's much better.  For example, if your opponent checks into you on the river, he's shown weakness, if you bluff you can probably take the pot; but - he might be slow-playing a good hand.  If you have something decent you should just check it through.  Bluffing makes you feel like you're getting more chips in the pot, but it's not a bigger win!