A good, dead room. A good, bright microphone. Proper placement. Then a combination a "squashy" compressor for energy (like 1176 in "Nuke" mode) then a LA2 for overall levelling. There might be some FX, like reverb, if it fits the track. I used to over-do reverb before since I didn´t know better. But these says I like to keep things a bit drier. Dub is nice. I prefer three then. One in the middle. then one dub in each speaker, maybe not compress them as much, then you get more "life" in the dub. Also try a "Chuck" with some harmonizer like minus a few cent in left and plus a few cent in right. Fattens up one take like hell. But to dub with a little variation in pitch (Growling...that is) is also nice. I have also added a whispered version of the growl and added that as "air". Works nice when your throat is fucked. Happy hunting.