In my many conversations with apologists, Bible thumpers, and the extremely pious; I've found that the most effective way to "win" starts off by showing that the argument for a) the existence of god, and b) the god they believe in, require completely different proofs. Former apologist John Loftus writes about it in his blog 'Debunking Christianity' as well. Numerous nonbelievers, like Sam Harris, successfully use the same strategy.

The Proof for Deism

Arguing for the existence of god is called "natural theology", or deism. This god relies on gaps in scientific knowledge. We should have no problem on a practical level with this. Until those gaps are filled, this god will always exist. So, arguing against deism is basically useless because either way it won't matter.

The Proof for Christianity

But Christianity is something completely different. Christianity does not rely on gaps in scientific knowledge. It relies specifically on whether the Gospel accounts of miracle stories attributed to Jesus are true. Just establish this obvious distinction first.

Soon, I will place another post up for the next step, which is simply the basic argument for why we should reject Christianity. But if you want to watch a short version of it, here's how Sam Harris did it at the La Ciudad de las Ideas debate earlier this year.