1 Insist that "religion is illogical," especially if you've had no training in formal logic and couldn't tell a syllogism from a shibboleth.
2 Insist that you "just believe in one less god than you do." Ignore the fact that a Christian monotheist and a Hindu or Neo-Pagan polytheist agree that the supernatural exists, which puts them closer to each other than either is to you. And never agree that a Christian (or whatever) might concede that there is an element of truth in other religions.
3 In any discussion of religion, you must make at least one reference to Santa Claus, the Easter Bunny, the Tooth Fairy, or invisible unicorns.
4 Be sure to mention the Crusades, even if you know nothing of medieval history and couldn't give a concise summary of the events to save your life.
5 Be sure to mention the Inquisition and/or the Witch Trials, because if anyone claiming to be a Christian ever did anything evil, it obviously invalidates that religion.
6 If your opponent brings up the fact that more people have been killed by officially atheist regimes than in all the religious wars and persecutions in history put together (e.g. 80 million by Stalin, 60 million by Mao, 1-3 million by the Khmer Rouge), insist that they weren't "real" atheists, or that "communism is a religion."
7 If your opponent says that if bad atheists don't invalidate atheism, bad Christians don't invalidate Christianity, refuse to concede the point. Bring up the Inquisition or the Crusades again.
8 Claim that "science" somehow proves atheism. Ignore any claim that science can't do any such thing, since it operates on the basis of methodological materialism.
9 Claim that evolution falsifies Genesis. If Christians or Jews claim to have no problem with evolution, accuse them of denying what the Bible actually says.
10 Claim to know more about the Bible than "Christians" do. Ignore the immense implausibility, since Christians are likely to read the Bible every day and study it at home, at church, and on church retreats.
11 This is most important insist, as heatedly as any extreme fundamentalist, that the Bible must be read in the most naively literal sense in all passages, and therefore the tiniest mistake or scribal error invalidates the whole thing and any religion based on it.
12 Imply that your opponent is mentally ill. If he/she doesn't seem to get it, be explicit. After all, religion is a form of mental illness. Ignore the fact that theists are no more likely to score high on psychiatric intake tests than anyone else.
13 Two words: Richard Dawkins.