Lamb of God doesn't really count as a growl, it's more of a raspy cackle if you ask me. Growling is gutteral and it works pretty much just like any other vocal style. Push air from your stomach and aim for a lower pitch. Only real difference is your distorting it. The thing about it is if you're not careful you can pull a Chris Barnes or an Anders Friden and flat out ruin your voice if you use it like a toy.
I'm not too much doing exclusive lows anymore though, lately I've been working on my higher wails, so those are coming along quite well. But yeah, my lows are incoherant, but I can add clarity.
The Gods Made Heavy Metal, And They Saw That It Was Good
They Said To Play It Louder Than Hell, We Promised That We Would
Hulk Smash!!
Whatever you do, don't eat limes. A friend of mine ate a lime once and BAM!! Two years later. Herpes.