I've recently "come out" as an atheist to family, friends and coworkers, which is not unlike tiptoeing oneself into a pool of very icy water. One of the hardest things for me has been how to eliminate the use of the word "God" as exclamation, i.e. "oh God, Paula is such a drama queen!" or "God, that was an incredible movie!". Yes, it is a rather trite issue but it concerns me. I was raised a Baptist and live in Southern Virginia and even though I grew up with the notion that uttering the word "God" in anything other than prayer or in discourse directly involving the concept was grounds for a stern lecture about using the name of the "Lord" in vain, it grew to become a part of my personal vernacular and I really want to stop using it.
I use "god", in the same way I might use fuck, crap, whoa, shit... just another single syllable word with no context other than that of the sentence it's in. When I'm really agitated I might say "Jesus H. Tap-dancing Christ on a Pogo Stick!". While i'm not trying to offend any nearby xtians, I don't give too much of a damn if I do ;)
You could always go the South Park route and replace God with 'Science" as in: "Thank science you made it on time! :P
Seriously it's difficult to eliminate from your vocabulary. I have accepted it as a cultural norm, but am painfully aware every time I say it.
When someone lectures you on taking the lord's name in vain for saying "God", you might inform them that you've done no such thing. You've mentioned a title, but not a name. The term/concept of god existed before Judaism or Christianity. If the same person keeps any images of Jesus on the cross, you might counter-lecture them about images of Christ.
If you'd really like to hear some good lines for replacing religious expressions in your vocabulary, I'd suggest watching the show 'Futurama.' It's got some funny, interesting sayings in it that, with practice, are relatively easy to convert your religious sayings to.
e.g. Turning the exclamation, "Holy Mother of God" or something along those lines into the Professor's "Sweet Mother of Science!!"
There are a good deal of such examples, and you can basically pick and choose the ones you find relevant or entertaining. The show's not too bad, either ;)