I'm fortunate enough to have a cousin that's an opera soprano... Her advice: stay off dairy food for a week before performing vocals. No milk, cheese, etc. When finished, celebrate by going out for pizza. Another good idea, stay away from caffiene as well. When tracking, stick with warm water or green tea. Never, and I repeat NEVER, drink anything cold. It'll tense up the muscles.
Another thing, I'll usually let vocalists borrow my copy of "the Zen of Screaming" a few weeks before tracking begins. This usually helps big time.... ...I learned the hard way: I started working with one dude & things didn't go so well. I gave him a week off & let him borrow the video. 1 week later, he came back & it was like working with a completely different vocalist. Highly recommended!