Melissa Cross has created a DVD called "The Zen of Screaming", where she discusses vocal techniques for growling to prevent vocal damage. She even sticks a camera down her throat, and demonstrates good and bad technique, and you can clearly see how the vocal cords can get nodules if you growl incorrectly.
Most singers who use the normal metal growl are tenors. It's possible to growl with other vocal ranges. For example, I know a girl is an alto with an exceptional growl - better then most guys. I'm a baritone, and I growl in my band - it's just more of a Rob Zombie growl sound then a traditional metal growl. I love it - it emphasises the angry emotions in some parts of my music. That and it turns heads :-)
As for techniques to lower the toll on your voice - breathing is extremely important. If you breath properly your vocal cords seem to strain less. This applies for normal singing as well, of course. Only breathe what you need for a growl - don't force air through. And don't growl from the 'back' of your throat - that will only damage your cords.