I think a good method would be the socratic method. its not terribly confrontational, and you get them to see the problems with their beliefs all on their own.
for example, if someone starts preaching about their god, you ask,
"What is God?"
It sounds like an utterly stupid question, but I guarantee you most theists have not come up with a meaningful answer to that. Once they pin down a concrete definition of God, then you can drill into specifics. If they say God is omnipotent, you ask the pedantic question about whether or not God can create a rock so heavy even he can't lift it (Yes, its asinine, but if omnipotence were logical in the first place, there'd be a good answer). Or if they say God is loving, ask why there is evil in the world.
Basically open a dialog. There are lots of canned scripted apologetic responses to these things, so you want to take them off the script. Make them think for themselves. They will start to question their beliefs. I know I did.