I don't defend my atheism. I don't attack their religion. It's none of anyone's business what my personal philosophy or belief system is. The proof is in the pudding and the pudding is made out of your actions in life. I generally dodge direct questions with answers like: it's been a while since I went to church or I like football on Sundays or even I'd just like to keep it to myself so I don't get into any political or religious discussions that get everyone ticked off. If I do eventually get into it with people I try very hard not to offend. No mentions of the holocaust, witch trials, gay marriage, or anything to show religion is wrong. It will only alienate you from people who could otherwise be your friends. I figure religionists are like kids who still believe in Santa Claus, why ruin their Christmas just because you know it's your folks who put the presents under the tree?
I'm not much for being a proselytizing atheist (which really sounds like a contradiction in terms to me, anyway). However, if someone wants to proselytize, get in my face, attempt to convert, threaten me with hell, or whatever, I have no problem countering with whatever intellectual force is necessary. I may not be able to take such a person out with the skill or erudition of a Christopher Hitchens, but I will absolutely take my best shot at emulating him and reducing my opponent to a mass of quivering illogic.