Don't memorize theorems. That said, if you want to remember what a theorem is saying then there are a few things I find helpful:
Try it out in a computable example. If it's a classification theorem, pick some object and follow the steps of the proof on your chosen object.
Build examples and counter-examples. The theorem likely has some conditions where it applies and doesn't apply. Try to figure out what examples force the hypotheses of the theorem.
Try to remove hypotheses. Maybe you can't find counter-examples for the hypotheses of the theorem because there aren't any! See if you can tweak the proof a little to remove a hypothesis.
After you've gone through a few of these you'll find yourself much more familiar with the theorem and its proof, and (hopefully) you'll find it easy to remember it.