Before you sing, dont drink a lot of sugary stuff like pop or juice or even beer. Milk is even bad because of the mucus. What all this stuff does is coat your throat and mess up your singing ability. This is bad enough if your trying to practise singing, but even worse on a gig.
One cool thing my band's vocalist does is have a glass of hot water and honey with some lemon or lime juice in it (straight from the fruit, no sugar added). These are all great for your voice, I even had a glass just for fun and it really made a difference just in the way I spoke (I dont sing, I just wanted to try a glass).
Anyway, we do a lot of AC/DC kind of stuff, so she drinks this stuff whenever her voice gets strained, and poof, its all good.