Well, yes, take Zero's advice about doing the gutteral growls. Personally, I only use them when I am doing a straight up death metal growl, i.e. Mikael Åkerfeldt. I still have trouble with Stanne's growling style from Damage Done, but I am getting there. As for black metal, I use my throat for that... it helps getting the high pitched tone. I am pretty good with Anders Friden-style growls too.
All around, some tips would be to stop when your throat starts to hurt. Drinking any warm liquid while practicing will help prolong the time you have to practice without your throat starting to hurt. By "using your stomach" to growl... well, I'll try to explain. You really should be using your abdomen to push the air up through your throat. It's hard to explain, but that is what it feels like.
Hope this helped some of you folks...