Suck halls, don't speak if you can, have ice cream, hot chocolate (though, not too hot), soup (not too hot), and anything liquidy or creamy. Don't forget some medicine for this stuff. Don't forget some rest. If all else fails (and you don't have a solo) sing what you can and mouth the words.
*HELP* I just got a sore throat last night and I have a Choir concert on Monday...? *HELP*?