Dan,
I'm NOT an expert, but I've had some success just sticking
my SM-57 about two inches away from the center of the cone at sort of an angle. That is, the mic is at about 45 degrees off horizontal but the "tip" of the mic is directly inline with the center of the cone.
Aiming your mic closer to center will give you less of a bassy sound. Basically I leave mine in the center and never screw with it...I use the good ol' tone knobs on the amp, guitar, pedals, and mixer (sheesh) to get the right sound at record time.
Also, try not to run too many effects while recording and know that the sound coming out of your amp will not be exactly the sound coming through your headphones. Depending on what you're recording with, you've got a lot of crap between your ears and the amp. Effects like compression and reverb can be tweeked after recording.
Also, you'll probably want to record with quite a bit more treble and less bass than you're used to. It'll sound annoying and less "deep" while you're playing it...but when you add in bass and drums or whatever, you don't want your guitar to drown out the other instruments or be drown out. If you listen real close to most albums, you'll notice that the guitar is rarely as "deeply" toned as what you prefer playing at home.
Another thing you can try is a crappy radio shack microphone BEHIND the amp (if it's open). Pan
the SM57 hard right and the cheap microphone hard left and record stereo. The cheap mic will add a lot of high frequency that you're lacking. This of course requires some proper mixing. Watch the level on the cheap mic cause it DOES sound crappy when you can really *hear* it.
Slackmaster 2000