When I'm confronted by theists over why I'm an atheist, this is usually my response:
"When I look at the universe around me, with all of its wonders and spectacles, I fail to find any phenomenon that cannot be explained scientifically or that definitively require the invocation of a supernatural entity. Furthermore, even if some phenomenon were observed that required said invokation, I fail to find any reason for why said supernatural entity should take any one form over any other."
Rephrase the last sentence to fit the theists' belief, e.g. if they are Fundamentalist Christians, I would say that there's no reason for why said entity should be as described in the Bible.
Sometimes, however, I do question myself (I'm paranoid), and I wonder if God (as we've defined him over the millennia) really exists.
When that happens, I watch a few videos on YouTube. I have them now on my iPod, so I can watch them on the go. I recommend that everyone wtach these videos, as they are probably the most spiritual scientific videos you will ever watch.
Neil Tyson -- http://www.youtube.com/watch?v=0Ai-VvboPnA
Neil Tyson, on Stupid Design -- http://www.youtube.com/watch?v=C1cKD93W3yg
Carl Sagan's "Pale Blue Dot" -- http://www.youtube.com/watch?v=2pfwY2TNehw
"Pale Blue Dot" Remake (watch it even if you watched the one above) -- http://www.youtube.com/watch?v=47EBLD-ISyc
EDIT: Here, lemme embed the playlist: