I have been told that the most effective way to convert believers to atheism is to plant a seed of doubt and let that destroy their religious beliefs from within. In two easy steps!
1. Ask why they believe what they believe.
2. Ask whether they think people of other religions feel the same way about their religion and/or have had similar experiences. Alternately, ask why (1) indicates the existence of the god of their chosen religion (excluding all other gods).
I have tried this only once, but it was the only thing that got the other person to even consider my point of view.