What do you say if you're asked to a dance by someone you don't want to go with?

I haven't actually been asked yet and haven't been put in this situation, but I want to be prepared in case this does happen. What do you say if you are asked to a dance, but you were hoping someone else would ask you? If you say yes, then you lose your chances of going with the person you like, and if you say no you'll hurt the asker's feelings because technically you're free, but you don't want to go with that person. If you say you can't make it to the dance, you don't want the asker to actually see you there with another person. So what are you supposed to do?