The Shure SM-57 is a good mic for this application. I've found that people tend to get too fancy in placing it. The mic doesn't 'hear' quite the way we do. I think you'll find that if you hang it by the cord over the cabinet so that the 'ball' of the mic is just back of the edge of the speaker, it sounds pretty good.

This assumes that your amp is giving you the sound you want in the first place. Good mic placement will help capture your sound. It has to be there to be captured.