what kind of placement config do you recommend with that sm57?
I think I have seen them like 'draped' over an amp before.
I do a bunch of things. I find that the closer you get to the middle the more top end you have. If its a 4x12 I mic one of the top speakers. I avoid micing an amp on the floor, I like to get the mic off the floor by atleast 1-2 feet because of reflections that change the tone when its close to the floor. I like to mic the edge of the speaker but again your ears are always the best tool. If I am micing a combo amp and I find the tone is very top end heavy try micing the speaker from behind.. I find that sometimes micing the combo amp inside from behind gives me a warmer tone.
But again there are no hard and fast rules. I generally move the mic around or have a stagehand or monitor guy move the mic for me so I can listen to changes out front. Because an inch here or there can make a world of difference many guys just stick it in front and leave it then they end up eqing the crap out of it to make it sound right..
Spend time with placement you will be rewarded with good sound. As far as distance is concerned it depends on the mic and the sound your going for. Live I try and stay no more then 2-3 inches MAX away from the grill. I never put the mic directly on the grill... why? because it vibrates

Just take your time, but if your source sounds like shit fix that first I always get the lead guitar levels and rhythm levels worked out because I dont want a guy boosting his own leads on stage * that's what I get paid for *
I always get guitar players to sit in front of there amp at a lower volume.. and listen to how it sounds then for a few seconds I get them to crank it up so they understand the difference between low volume "hey it sounds great" to high volume "man its ripping my head off at 2k"
The more time you spend on the source the better your mix will be. Sometimes you just have to literally put a mic in front of it and call it a day it sounds good already... when I am dealing with "old pro" players who know what they want I "deal with it" with out asking for changes. Its always a fine line between messing with the guys vibe and making sure things sound good. In the end everyone needs to work together to make things sound good.
For the 57 you need to work that mic very close to the source and remember its very directional way more then a 58 Sometimes I find that a 58 sounds better

Because its got a wider pattern.
My favorite mics for guitar are the Sennheiser 409, AT 4050, Shure SM7 SM 57, SM58. And Sennheiser 421, Royer Ribbon mic, AKG 414 and I also like the Akg 125 its an old dynamic mic. But I heard a Peavey mic sound good

so I guess that tells you something about placement and the source.
Chris