Are you familiar with the Five Proofs of the Existence of God as formulated by St. Thomas Aquinas? They're pretty stout logic if you can articulate them well. I find the fifth argument to be exceptionally convincing, and most atheists upon whom I have used these arguments agree. It's undeniable.
http://en.wikipedia.org/wiki/Quinquae_viae
Also, Pascal's Wager sometimes has some emotional appeal. Basically, the wager shows us what is sensible in terms of belief. Either God exists, or he doesn't. He can't sort of exist or kind of exist. Either he does or he doesn't. Our options are either to believe or not to believe. From these two things we have four outcomes.
Believe in a God who exists. (When you die, good.)
Believe in a God who doesn't exist. (When you die, big deal)
Don't believe in a God who doesn't exist. (Once again, when you die, big deal)
Don't believe in a God who exists. (When you die, big problems)
This shows that there's only one sensible solution. Of course, it's a very low motive--it appeals to our innate instinct of self-preservation, but that sometimes proves effective in motivating people. It also eliminates agnosticism as having any intellectual or moral significance whatsoever.