Tips for being a good atheist
When it comes to religious debates, it's often tough to stay level headed and civil about things. Knowing who to argue what with will go along ways to keep the name calling and yelling out of the conversation.
Know your target
You know that 50 year old guy who's gone to church 5 times a week since he was 4? He's probably not a good person to debate with. You have pretty close to no chance of changing his mind, just as he has no chance of changing yours. There is very little chance of that staying civil. The whole point of a debate is to, even if just a little, sway the other person into your thinking.
So who should you debate with? You want someone who is more curious than argumentative, someone who maybe asks you how or why you are an atheist. Whether they admit it or not, chances are if they are asking, the probably have at least a little doubt. They may just be wondering, but there is also a much higher chance of that conversation staying civil.
Know your topic
From my experience, 9 times out of 10 just stay away from the creationism versus evolution topic. When their argument is to open a jar of peanut butter and show you there is nothing living in it, there is pretty much zero chance of having a logical conversation with them. There are a few exceptions, but for the most part, stay away. When someones argument is something along the lines of "I know the bibles true cause it says so in the bible," just get big eyes and say "your right, I see there light there is a god" and walk away. All any religious versus atheist debate boils down to is logic versus faith. When they take away your best weapon by completely ignoring all logic, there is nowhere for that conversation to go but downhill.
Good debates come when the other party is being at least a little logical. As an example, whether you believe in it or not, the argument that humans are complex so must have been made by a higher power is a pretty good argument. Things that we can't explain but religion has an answer for are topics you want to aim for. They will keep it a polite talk, instead of a bar fight with words.
I know you are but what am I
It's human nature for atheists, whether they admit it or not even to themselves, that most of all religions are filled with ridiculous ideas and you have to be stupid to believe it. Whether you feel that way or not, it doesn't need to be said. An example I like to use is there is a group called the flat earth society, look it up if you could use a laugh. Looking through their site, it's almost impossible to not think they are all idiots. Just because you're thinking it doesn't mean it needs to be said. Even the most civil conversations will turn bad quickly when you start calling names. Also, stay away from calling their beliefs stupid, even though you don't say, or perhaps even mean, it doesn't mean it won't be taken as you calling them stupid.
I spent many years ashamed, for lack of a better term, of being an atheist. Don't be. Be proud of who you are and what you believe, just be proud in a respectful way. Religious debates can be extremely fun, as long as it's with the right person, on the right topic, and done in the right way. Don't give the rest of us a bad name. Finally, know the bible. Won't go into too much detail on that, but keep Matthew 5:17(NIV) handy. Happy debating.