I've tried a lot of lines and the one that has worked best is when they go to pay at the end, I'll say something like, "Okay, we did 4 songs, so that's at least $100" (dances are $25), and I'll virtually always get a tip on top. Usually 20s.
I think it comes across less as asking for a tip, and more like, "You're better than the bare minimum, aren't you?"
