Great advice from Jerry Coyne (longish article at the link):
Our greatest weapon against religion, and especially against theologians, is this question: What evidence do you have for your claims? Theology will wither, and with it religion, if we just keep asking that question . . .You know, he's right. It's the only thing we ever have to say to religious twits. What evidence do you have for your claims? It's a conversation ender because they have no evidence, not one whit. Sweet and simple. I like it and will try it on the next believer foolish enough to talk to me. Moo-ha-ha-ha!