Most of this has been said, but here's my philosophy.
1. Obviously good player,then guitar, then amp.
2. Get the cabinet at least away from walls. I sometimes get it off the ground but not always.
3. Test all the speakers! On a 4X12 there's usually 1 speaker that really stands out. What I do is I grab 4 57s and put one dead on the middle of each cone. Feed them all to the same pres, match levels....and listen. It's amazing how much better it can sound if you start with the right speaker.
4. Check phase when using multiple mics, and check phase especially on MULTIPLE CABINETS!
5. I don't know if anyone else does this, but I found (especially with HEAVY stuff) that if I track my levels a little lower then normal in PT ( say.... -10?) that it actually ends up sounding way better when I quadtrack. The lowend of the guitars doesn't eat up all my headroom.
6. I'm a big fan of the ross hogarth (first person I saw do it) 57/421/121 combo. I'm a HUGE fan of the chandler pres, and can't wait to try the new germanium