As a former theist myself, I have found that the most effective strategy is to argue with them from within their worldview and by their own rules. The reason for this is that they (in my experience) flatly reject any evidence external to their religion.

So, rather than discussing the existence of God, assume God exists and then point out that if God were a human, we'd call his motivations needy and depraved. Rather than bringing up outside savior myths which influenced the gospel writers, assume that the gospel is divinely inspired and then point out that it was divinely inspired in four different (contradictory) ways in the story about Mary visiting Jesus' tomb. There are plenty of these.

In my experience, this method is far more effective than outside evidence, but tends to lead to dismissals of logic itself, which is where I get frustrated. I try to explain that logic is embedded in language and thought, that to dismiss logic uses logic (albeit poorly), but I can't seem to make that point stick.