I think their are several approaches that are commonly used, Unison, multi tracking the same part 2 or 3 times, like L said, different guitars, amps, pick-ups settings could help alot, also mike placements, Jimmy Page used the old distance makes depth formula. Octave, same part, an octave higher or lower. Intervals, most common 3rds, 6ths, occasionaly 5ths or 4ths, something Neal Schon did on Journey"s Infinity album ( I think it was Infinity ) on the song Ides of March?, memory is a tricky thing sometimes, was to record the root, then the 3rd, then the 5th, so you are playing a triad, which will sound quite thick, Something you might want to check out is Benoit's webpage and his piece called Renaissance, if that's what you are looking for, I'm sure he would discuss it with you...