A method that I've been toying with: dissect the proof into multiple simpler proofs, then dissect those even further if necessary. For instance, if you're proving that all X are Y, and the proof proceeds by proving that all X are Z and all Z are Y, then make 3 cards:
That said, you should of course be completely certain that memorizing proofs is worthwhile. Rule of thumb: if there's anything you could do that would have a higher ratio of awesome to cost than X, don't do X before you've done that.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.