I've auditioned for tons of stuff...my advice to you is just to listen to as many soundtracks/cds as you can and find a song that feels "right".Also, don't sing something a million people are going to sing (ie: "tomorrow" if you're auditioning for annie.)
This may sound kinda weird, but that's as much as I know (it's worked for me, at least). It took me about two years to find "my" song. I got it from "Whistle Down the Wind". Great soundtrack if you're looking for a song. Lots of stuff/styles and not alot of people (at least around my area) know about it, so you've got a better chance of keeping the judges' intrest. Good Luck!
Was that a fat joke?