I sing the brutal death style so I have to pretty much go for the growls all the way through.
The best way to do it is start off making 'grrrr' noises. Like your taking the piss out of a dog. They sound laim but are the direction that you want to head in
(HINT: Dont do it infront of anyone) Once you've got your basic 'grrrr' down pat you'll want to make it sound like you've got some balls. Dont do anything that stresses your throat, you've just fuck yourself over and not be able to do anything. Ever.Use vowels as your basic to start with, play with the sounds. At this stage your not trying to sing, your trying to get a good sound. The best growls to use to start off with are "Arhhhhhh"s "Grrrrr"s "Riiii"s "Youuuuuuu"s.
Once you've got that basic gravel sound go for the variations with the lips, smaller the opening of the mouth the lower the sound. When your mouth is closed more you have to force more air out to get it loud enough. With combination of stomach and throat you should be able to get a good sound.
I was going to write more but I just realised I cant be stuffed and its different for everone.
Ohh yeah, for practicing I usually run through my ABC repeatedly as fast as I can (See how many times I can say it all the way through in one breath)