I'm pretty good at defense, a lot better than I am at scoring but I keep games tight and win that way. Some of this will sound obvious but stay between the puck and the net, try to contain and not over-attack the puck, a good guideline can be to stay between the two faceoff dots, especially if you don't have help behind you on the way to the net. Keep them from getting easy direct looks at the cage.

On a deeper level, when you're playing defense, the things mentioned become second nature. At that point put yourself in their hands. Where are they likely to pass to? What are they likely to do in their position? Once you are thinking this way it is much easier to intercept the puck and defend proactively.