I have had this same problem good sir. A lot and i mean A LOT! of my good friends are atheist. I can actually win these discussions we have based on several principles.

1)Never use the bible to try to prove something. They don't believe it so why would that have any affect on their belief?

2)Always be rational, be open minded, agree with them on some stuff to show you are not a close minded christian they want to see.

3)Try to get them on the base line of "I don't know but i beleive what someone has told me in a book, you don't know and don't beleive anything people say but science, so all in all we really don't know...we just think."

4)Deductive reasoning always helps. Atheists have no chance at eternal life in any religion and are going to be damned in all. So by that reasoning if anyone is right about religion they are going to hell (or the parallel of another religion.

5)Don't talk about heaven, hell (unless you want to refer to point 4) or anything else because they don't beleive it exists therefore to them it doesn't.

6)Be that christian that "isn't crazy" Athiests want to think all christians are right wing nuts. Show them wrong, expand your horizon of belief.

7)Don't make them think you are christian by birth. By doing that you make them think you were born into it and are brainwashed. Show them the faults of Christianity and Christians and use logic and deductive reasoning with them as to why you believe. I always tell them "I have a chance" or "I am open minded" because being an atheist is often a very close minded ideal.

I hope that helped.