The trick is to really control the performance of each note. The use of slides, muting, vibrato, bends and harmonics all contribute to the sound and its important to thoroughly understand and master each technique. I also personally prefer the neck pick-up for notes above the 12th fret, although that isn't a set rule. I believe its important to really play the guitar as hard as possible. In my experience, playing with conviction really lends itself to the aggressive sound sought after in heavy music.