"My bf and I have a pact that we don't dance with other random guys/girls, no offense but he's not around and I don't want to betray his trust"
"Sorry but I'm a bit tired, think I'll sit this one out"
"Sorry but I'm here keeping a friend company and don't want to just leave her alone here" (if you're in a club, etc)
"I was dancing for a long time a while ago, I just want to relax for now, sorry"
"My friend's getting over a bad breakup and we made a pact that this night would be an exclusive girls' night out, so sorry but I have to be there for her right now..."