I've been listening to Opeth since like 10th grade and I eventually learned how to growl, now 6 1/2 years later. My advice is to just practice making the sound in whatever way you can, don't worry about making the words. Also, having bad sinuses (or drinking heavy mixed drinks with OJ) helps a bit. It'll come from the back of your throat.
I love growling along to Opeth, but I don't have near the lung capacity developed, because my growling takes a lot of air to sustain. It's fun to try to emulate the growls on, like, The Moor's opening growl. I know it's a few growls overlapped, but I like to think one of them is a big 10-second growl (or however long it is).
As for regular singing, anything from Damnation or Blackwater Park. Face of Melinda and Harlequin Forest get honorable mention.