Sorry, but I have to disagree with Alice on one point - I've always been taught NEVER drink milk or eat dairy products (including chocolate) prior to singing - supposed to make mucous production more likely, and I have noticed that this sometimes does happen when I disobey the rule. Not an attractive addition to most performances, hacking & spitting one's way through the song! ;-)
A warm salt water gargle does help with colds & sore throats, or hot lemon & honey water, and I have had others recommend the glycerin spray she suggests. I believe a catalogue called "The Music Stand" offers it.