However nice it may be to watch a live debate in front of a crowd, you should never agree to this unless you are well prepared and posses a tight grasp of the topic. Christians that agree to this type of venue are usually willing to do so because they have been studying the arguments against their position for many years. Very often teaching classes or holding study groups on the weekends when they should be getting drunk and watching football like a normal person.
For this reason, I suggest confronting Christians when they are least prepared, like in the drive thru line at Burger King or while they're preoccupied on the toilet.
I remember one sunny August afternoon when my neighbour was holding her child's 9th birthday party in the backyard. I waited respectfully for the candles to be extinguished and gifts revealed, before popping my head over the fence and politely asking the first child that walked by, why it was his God gave my Mother a brain tumor the size of an elephant testicle. My stealthy approach caught him off guard, leaving him unable to answer my question. Instead, he ran to his mother crying, like the coward he was. She overreacted (as you might expect) and threatened to call the police unless I left them alone and put some pants on. I complied with only one of these requests, I'll let you figure out which it was.
My point here is that surprise is a very effective weapon in debate. Challenging someone on their silly beliefs when they are least expecting it is a sure fire way to win the argument (and sometimes get punched in the neck).
Know Your Topic:
The river of bullshit that flows from the mouths of Christians is too vast to be crossed in the dinghy of reason alone. You must recognize where your strengths and weaknesses lay and utilize those areas to there maximum potential. Never start a debate unless you have a firm grip on the topic to be covered. For instance; if all you know about Jesus is he was born to a virgin and nailed to a pole, do not get into an epistemological argument over his divinity. However, if you've read at least two chapters of the bible and seen Kirk Cameron's 'Left Behind' series, you probably know more about Christianity than most lazy Christians, so go ahead and start a fight.
Use The Bible To Your Advantage:
Most people who profess to be Christian have not actually taken the time to read the book they claim their God wrote. Because of this I have found that you can say just about any crazy old thing as long as you preface it with "Deuteronomy 18:21" or "Leviticus 6:11," etc. An example of this would be;
"If your God is so concerned about the welfare of humans, why did he say in First Thessalonians 2:14, that all those who possess eyebrows that come together in the middle, should be put to death?"
Those of us that have read First Thessalonians, know it says no such thing, but most Christians don't know what a Thessalonian is, let alone that it's a chapter in their magic book. Nine out of ten times they will accept your nonsense as fact because they don't want to admit they have never seen or heard that passage. This may also inspire them to actually read the bible, where they are sure to find far worse statements by their God than anything you can invent.
Don't Be Afraid To Call Their Mother A Whore:
I'm still not too sure this one is a good idea, but I'm going to suggest you try it anyway, because I think it's funny.
Choose Your Opponent Wisely:
If there is one thing I have learned from my many years of watching sports, its that Jesus plays a large roll in the life of athletes. Every time someone wins a medal or championship of some kind, they are sure to thank God, Jesus, Count Chocula, or whomever it is they fawn over on Sunday mornings. Deity worship is very important among large athletic men. I mention this because it is important to know just how dangerous it can be calling Jesus a "medieval fuck-wad" among certain company. One good rule of thumb to follow is, if the person you are speaking with has cauliflower ear, or a tattoo of a cross on their neck, keep your mouth shut and move on.
Use Profanity As Much As Possible:
For some reason, many Christians give special powers to certain words, I have even been told that saying the name of their saviour during orgasm will produce cloven-hoofed babies, but I can't confirm this.
Because of this infantile fear of "bad" words, I suggest using them as much as possible to befuddle your opponent. If you can confound them with a fuck-fuck here, and a shit-shit there, here a fuck, there a shit, everywhere a fuck-shit, you will surly win the day, regardless of the topic.