I used to actually scream harsh vocals but I couldn't speak properly for two days after and it didn't sound like I had any power behind it no matter how much effort I put into it. What I did was really starting to work on using my gut and really push the air out instead of screaming and it resulted in two things. First of all it sounds so much more powerful when recorded and it's also easier to record because the volume is much better, secondly my voice takes close to no beating at all in the process, I can scream for two or three hours without any hurt at all. I guess the best thing is to practice clean vocals first to you get a proper way to sing (using the gut) and apply the pressure to make it harsh later. Good luck! If you want to hear a smaple of my "singing" go to:
Http://www.myspace.com/blackanima
I guess I'm not the right person to give advice he he he!