Heads up in poker is the equivalent of one on one on the basketball court. You are facing only one opponent. But just as in basketball, there are some intricacies of the game that will help you gain the advantage over your opponent.
For one, you should be aware that deception will be used much more often heads up than in a regular game. Not every hand is going to be a good one, yet if you play only the good ones, your opponent will ultimately win more often than not. So you need to deceive your opponents and play as if you were receiving more good hands than bad ones.
Another thing to consider is that in heads up play at PokerStars.com, you will have worsened pot odds on most hands. Draws are not going to be as beneficial to you here because you have fewer opponents in the pot, thus making pot odds all but disappear. You will not want to chase four flushes or gut shot straight draws simply because you will not hit them often enough to make paying your opponent off worthwhile.
Lastly, aggression is your friend in this instance. You will want to be the bettor more often than the caller. But you do need to realize that your opponent will most likely be following the same philosophy. When you have good opportunities, raising a hand with a low pair, or even sometimes an ace high, will be your best strategy. This is simply because in the latter stages of the game, your opponent cannot take on the risk of calling you with nothing in his hand.