there are three main things that really dictate the sound you get when recording guitars. Unfortunately these things are usually out of your control as the sound engineer.
They are (in order of importance):
- technique of the player
- Guitar used
- Amp and Speaker Cabinet used
Pretty much everything else is minimal. For example, what mic/preamp combo can make a fender strat through a fender twin 2x12 combo amp sound like a Mesa Boogie Triple Rectifier through a rectifier 4x12? Answer... none.
If you are looking for heavier tones, you have to find amps that give you those tones. A line 6 amp (i've recorded them before) is not going to cut it.
Also, you can have an amazing amp and guitar, but if the guitarist isn't that good, the sloppiness of the playing will ruin the tone. It's hard to realize this until you start recording some phenomenal players and you suddenly realize "wait what am I doing different, this sounds amazing." And the only thing different is the player. Even when recording direct with something like bass the player makes a huge difference in the tone.
Anyway... once you get those things in check... then a lot of these other suggestions people are making will help. One big one, turn the gain down a little more than you would think. When double/quad tracking guitars, if you set the distortion where you like it for ONE guitar pass, by the time you stack it and combine them all, it will be mush. Dialing back the gain/distortion a little from where you like it, then do your passes, yields the best results most of the time...
multiple mics are also usually a good choice. You have to make sure they are all phase aligned. Do do this, make sure they are all the EXACT same distance from the cabinet. When you combine them and blend them, you get a thicker, beefier sound. Using different mics to highlight different characteristics can be a good choice. Use a kick drum mic or ribbon mic to capture the low mids and bass, and a 57 or MD421 to capture the mids and high mids. Something like a U87 or any vocal condensor mic will get a lot of the top end that the other mics dont. blend to taste.
Also realize that treble comes from the center of a speaker cone, bass comes from the edge. Moving a mic closer to the center of the cone makes it brighter, moving it closer to the edge makes it darker. using a kick drum mic for bottom end works better when placed on the outer edge of the speaker cone than dead center, for example...
speaker cones are not uniform in freq response until you get out PAST the distance equal to the diameter (a 12" speaker would be 12" out... and so on). That means, as you move the mic around in front of the speaker within that distance three dimensionally (in, out, up, down, left, right), you'll find pockets of accentuated mids, pockets of highs, pockets of lows, etc. all sorts of phase shifting and phase interference is going on in front of the speaker causing all kinds of comb filtering. some of that comb filtering can be pleasing tones, others can be unpleasant. Moving the mic 1" in any direction (including in and out) can dramatically change the sound you are getting. so experiment...