Texas Hold'em: Deceptive Plays

Adding deception to your poker game is very important because it makes you less predictable.
If your checks always mean you're weak or your bets/raises always mean you're strong, the more observant players will have an edge on you.
Some examples of deceptive plays you can add to your arsenal:
The Free Card
When you're in late position or last to act, you can raise with a drawing hand on the flop.
This will likely make your opponents check to you on the turn, thus giving you the opportunity to check (if your hand does not improve) or bet (if you hit your draw). This will save you money if you don't improve, and make you money if you hit.
However, this move will backfire when you're re-raised on the flop. In these situations, it will cost you money but it remains a good play marked cards since you obtained information and have a draw to a better hand.

The Check-Raise

When you hold a good hand and it's your turn to act, check in the hopes that an opponent will bet so that you can raise when your turn comes again.
For example, you're in early position and have A♥ Q♠. The flop is A♠ Q♥ 6♠. You check and three players in middle position also check. A player in late position bets and you then raise.

The reason for check-raising is to make it too expensive for the drawing hands - such as a gutshot straight draw or overcards - to call.
The check-raise from an early position also gives you the initiative in the hand. If they still call, at least you've obtained information regarding the strength of their hands and forced them to pay as much as possible for trying to outdraw you.

The Semi-Bluff

Semi-bluffing is when you bet or raise with a hand that's not likely to be the best (at the moment) but you have many outs to outdraw your opponents if you get called or raised - although you're actually hoping to win the pot right there.

For example, you're in late position holding J♥ T♥ and the flop shows K♠ 6♥ 2♥, thus giving you a flush draw with nine outs.
There are three other easy cards tricks players in the pot and they all check to you. You bet without having the best hand but since they all checked, they indicated weakness and might fold pocket pairs, a pair of sixes or twos.
Even if you do get called, you have nine outs to the flush and maybe an additional six outs to win if you hit a J or a T, which makes 15 outs in total. If called and it's checked to you on the turn, you have the option of taking a free card in case your hand did not improve.

The Slow-Play

When you have a strong hand, it's sometimes correct to slow-play. This means just checking or calling on one betting round with the intention of betting/raising on later rounds of betting.

In Hold'em this is a very common play on the flop because you want to lure players in and raise on the turn or river where the bets are doubled.
This tactic can easily backfire, though, when you let your opponents take free cards that can beat your hand. Many players slow-play too often and lose pots they would have won had they not slow-played.
When this happens it is mathematical disaster, since you lose a pot you would have won had you bet/raised, and you have to pay off an opponent who has you beat.
In general you shouldn't slow-play when one of the following criteria are met:

1. A free card can beat you.
2. A free card is not likely to give your opponent a second-best hand.
3. There are many opponents in the hand.
4. It is a large pot