Avoid multi-mic techniques until you get a feel for how to place a single.
57s (or any other cardioid dynamic) are picky SOB's. You can get a great tone or you can get garbage, and the difference may only be a few MM.
Experiment, experiment, experiment!
My go-to placement for CD mics on my sealed cabs for heavy rythym tones is the edge of the cone. I work from there to find what I want, but I usually don't have to go too far.
Cardioid condensors are a whole 'nother animal. To my ears they sound better for solo work when placed at a greater distance. The mics vary so much it's hard to give a base placement, but generally you'll want to be a bit farther back with a condensor. This is especially important with large diaphram mics as placing one close to a sealed cabinet at high volume could damage the mic.
Then we have omnis. IMHO they sound better than any equivalent cardioid at close range, but they pick up everything. Sometimes that's a good thing, and sometimes not. Depends a lot on your room. Gobos can help with mid/high early relfections.
Once you get a feel for how the different mics react then you can move on to using things that are normally considered "bad" (like proximity effect, comb filtering, and phase cancellation) as advantages. This is where the multi-mic technique comes in.
You have to know what these things sound like before you can conjure 'em up at will.
Again experimentation is the key, but you need to understand what's causing the difference between what your ears hear in front of the amp, and what's coming through the monitors.
