Actually I think there's a few factors which are just about equal in importance..
1. I stopped using my cheap condensor microphone for guitars (which in combination with my amp which wasn't exactly the best anyways didn't produce the thickest sound) .. now I go line in through
my BOSS GL-100 like I mentioned above.
2. Studied alot about EQing and finally grasped the concepts I needed to understand before I could get a nick thick guitar sound every time.. Did one heck of a lot of tweaking my BOSS and comparing to cd's I liked.. One thing that really helped me was using the Fostex between the BOSS and my PC to control some extra EQ.. ie I would turn up the bass to maybe half on both the Fostex and the Boss, turn the treble on the Fostex to full, and totally OFF on the BOSS.. This gave me a much more "pleasing to the ear" sound,.. warmer and not nasally like it was before. I followed some advice I read about not going for the "scooped" mids, which honestly I never liked anyways.. so I put my mids on the BOSS to half more or less.. In Cool Edit Pro, cutting everything under 100khz to make room for bass and drums, definitely..
3. Listening back to old tracks, the drums pretty much sound horrid in retrospect.. I found some better samples and tried to make them match cd's I owned as close as I could. 1 big step was to ditch d-lusion
drumstation and learn fruity loops pro. Much less of a hassle for exporting loops and organizing them, not to mention tweaking individual drums and adding the proper reverbs for realism.. also the "humanizing" options are such a bonus. I love it.
4. My playing and mixing technique. It's been half a year and obviously these have improved somewhat. I'm going for the "each ear has it's own party" kind of thing.. The guitars kinda play off each other, left and right, good harmonies and whatnot, makes it interesting, and it's clear and crunchy enough that it still has that "wall of sound" effect. I despise the idea of layering rhythm guitars 2-3 times.. For this style, it would do nothing but muddy the mix I think.
Well, that's about all I think!