Many rationalists operate with the misconception that discussions should always be debates about the data points, not realizing (or caring) that religious belief often has deep emotional and/or cultural roots, and sometimes having little regard for the proper time and place to engage a believer. Accosting someone's deep-seated religious identity at constant and full volume actually produces the opposite effect of that which is intended, so while we may walk away with superior feeling of victory on the facts, we also may have actually prompted the reinforcement of bad ideas in another.
I'm a fan of relationships. I have my atheism t-shirts, books and websites, but I almost never brandish them when I first meet someone. It's important that they get to know me, my personality, my character, and that we develop a rapport, much like two strangers exchanging small talk about the weather before broaching, say, politics. Once the believer has seen me (and hopefully come to like me) in three dimensions, it becomes much harder to dismiss or caricaturize me as untrustworthy, unpleasant, or even evil. We become friends before we become ideological opponents.
After that, I never insult them. I try not to insult their precious belief, but I'm unafraid to declare that I'm skeptical. I use "I" instead of "you" as often as possible, which helps to keep them off of the defensive. And I ask lots and lots of thoughtful questions.
Granted, my questions often nudge them into a dizzying display of apologetic acrobatics, but they're helpful in planting seeds that may bear fruit in the days, months and years ahead. And because I genuinely care, they almost never detach, shut down and declare us enemies. We had a pleasant exchange in an atmosphere of respect and safety, and that's where minds are changed. —Seth Andrews