Putting all of the "social mechanisms of conversion" ideas together, it seems to me that the best way to help de-convert a religious person is to welcome them into a non-religious social circle, thus giving them the social freedom to explore their doubts without fear of total ostracism. (The group doesn't even have to be mostly comprised of non-theists, just not comprised of people who all belong to similar religions!) And if they bring up their doubts to you, it's a good time to talk.

And the best part is that whether or not it changes anyone's mind, you've just gained a friend.

(The flip side is this: avoid joining social circles where a majority are devout followers of the same religion- it will mess with your head...)