How to upkeep and maintain voice for screamer in /metalcore band?

I have been screaming for a few years now, but i still can't scream all day, every day. I use fry and power fry screams, because false and death are not options for me. However, I find that sometimes I can scream for at most 20-30 minutes before I begin to lose the ability to do so. If i try to push for it, it still doesn't work, and I'm not interested in vocal damage. Basically what I want to know is, what are good exersizes to do that maintain your screaming voice? What foods and beverages are conducive to my screaming voice? I have heard some people say Monster Energy drinks, but they are loaded with sugar. Most people say to sip lukewarm water, but while it is soothing, it doesn't really improve anything for me. Other people swear by eating skittles when screaming, and yet even others swear by singer's grace spray for your voice before a show. Ive heard that smoking a cigarette just before screaming can do wonders. Any input, Yahoo?