SM57s generally works best really close, almost touching the grill. Get a flashlight and pinpoint exactly where the speaker is. The 57 is usually placed centered with the speaker, or just slightly at an angle. Even a centimeter difference can drastically change the recorded tone, so have someone help you with placement.
You can usually find a good starting point by listing through headphones while nothing is playing, trying to find the point where the mic picks up the most noise. Finding the "sweet spot" depends a lot on room interference and equipment, so there's not much more I can tell you. It just takes a lot of trial and error.
Also, when you're setting EQ and gain levels, get level with the amp. If the amp is on the ground, but you're standing up, then what you actually hear and what the mic "hears" are completely different.
Try recording with slightly less gain than you think that you need. This won't make your signal less powerful, as you might first think, but will eliminate some of the fuzz.
When all else fails, there's always post-recording tricks like using EQ to boost the lows and highs, scooping out most of the mids, adding cab modlng or a slight reverb, and so on.
Most of it's just experience and finding what works best with your equipment, but if you have any questions, then I may be able to answer them or point you in the right direction.
...hope that helps.
