After looking back on my own deconversion from Christianity, I've realized that if someone approached me during the days that I still believed in the Christian faith and told me that there was no god, I would have balked at them and politely told them to go to hell. Now that I'm a devout Atheist, I often asked myself how did I go from being a Southern Baptist Christian to a rational, scientifically-minded Atheist.
It wasn't a billboard that converted me. It definitely wasn't a bunch of Atheists knocking at my door every Sunday morning to preach "The Good News" of Science and Reason. In fact, I can trace my deconversion down to one and only one thing: a glimmer of doubt.
Now, for most Atheists, this isn't groundbreaking or shocking news at all. I think we've all had our moments where we realized that something wasn't right about the religions that we used to believe in our younger years. The problems only start to appear when we try to "deconvert" theists head on by directly telling them that everything they believe in is a lie, regardless of whether or not what we're actually telling them is 100% true (and it is indeed true, and backed by science). You can't tell someone that all their beliefs are lies without expecting some sort of backlash. You can't tell a theist that their invisible sky monkey is a fabrication written by a bunch of 5000-year old desert nomads and expect to get a decent deconversion rate--it's just not going to happen.
In my opinion, the only reasonable way to "deconvert" someone is to start making them question various beliefs in their faith. For example, I asked one of my Catholic friends who visited me over the weekend about going to Catholic Mass every Sunday. I asked her if she were to commit a crime and confess it to a priest, would it be considered ethical? Why would any institution ever tell you that as long as you confessed the evils that you have done (no matter how terrible the act) every Sunday, you would be forgiven and then you can do the same thing again the next week, and be forgiven again? What if I were a mass murderer? Does that mean I get to kill a fresh batch of people every week and be forgiven again and again every Sunday? What about all the people that I supposedly killed as a mass murderer? Would it be fair for them if I got to go to 'heaven' just because I confessed the Sunday right after I killed them? Needless to say, I didn't make an instant "deconvert" out of her--but what I will say is that sometimes it only takes a little bit of doubt to get others to seek the truth for themselves. In her case, that little bit of doubt that I introduced was enough for her to question everything else about her faith.
Of course, this approach doesn't work with everyone--there are some people who are just way too religious to accept any form of (il)logical reasoning outside of their respective religions. What this approach does show is that the best way to get people to rethink their perspective about whatever religion they believe in is to have them question their beliefs from a very rational, non-judgmental perspective. The worst way to convince people that you're correct is to get into an argument and then rub it in their faces about how wrong they are. I can't quite put my finger on it, but there's something missing about the current crop of anti-theists and the whole "Atheist" billboard campaign. Telling them that there isn't a god isn't enough; there has to be some sort of message that shows them that they really never needed a god in the first place.
TL;DR; Charm is a far effective means for deconverting theists into Atheists; Telling them they're wrong just makes you look like an asshole.