Heads up is a poker game where there are only two players. This situation can occur when all other players have lost their money but there are games where you start only with one opponent from the beginning. The blinds usually raises fast in heads up games. The winner is the player who gets all the chips, of course.
The profitable playing behavior at heads up games is different from the standard poker game. There is very little chance that your opponent has good cards, so if you have a pair or just one high card you can win. You can also do all-in with only one high pair and have a good chance of winning.
Lots of people agree that you should forget the check option in heads up and play like there are two options: raising or folding. This is especially true when you are the blind: you should not let the other player use his cards only for the actual value of the small blind.
As the blinds raise fast, the small differences between the two players' chips are not so important in the first half of the game: it will change regularly who has more money. Because of this nature you should play with all the cards you have in the first half of the game: they are like free hands.
It is very important to know your enemy's strengths and weak points. Does he bluff often? When will he raise? What amount of money need to be raised in order to get him to fold? If you know your enemy you can use his weakness to win.