It's pretty easy to beat a mediocre player heads up. Basically, you just manipulate them mentially. If they are weak, push at them to steal blinds. If they are aggressive (and the blinds are small) roll over and trap them.

Against a good player - they will be looking to do the same to you. So you have to be unpredictable. The best thing you can do is to screw up their read. The reason is simple - in a heads up match, you want to get all your chips in with the better hand - which means you have to play HIS cards more than your. So conversly, he wants to play YOUR cards, not his. Which he can't do if you are constantly shifting strategies.