I found this great video by Devin Townsend of the band Strapping Young Lad. I saw his band open for Fear Factory about 5 years ago and thought they put on a really good show. When I was researching "heavy sounds" I happened to find this video. He has a great sense of humor and gives some great insight as how he achieves a "heavy" sound on his guitar.
1) To get a heavier sound it matters how you hold your guitar pick.
2) How you attack with your pick will greatly effect the heaviness
3) Don't pick strings strait up and down but instead use the "James Hettfield method" of letting the pick drag/scape across the strings slightly
4) Palm muting the strings is a very effective in achieving a heavier, percussive sound
5) When playing with drummers, it helps when they play a little behind the beat, it gives the sound a sense of size
6) Relax and playing will get easier
*Here is an example of Devin's heavy sound in action!!