I find the best way to train yourself to stop using these type of religious expressions (and to have some fun at the expense of the deluded) is to modify them rather than replace them completely, i.e replace "Oh God" with "Oh Thor", "Jesus Christ!" with "Cheesy Crust!" or "George Christ!" or if they are racist as well still say "Jesus Christ!" but pronounce it "haysoos", "God knows" with "Science knows", "God damn-it" with a plural "Gods damn-it", "Bless you" with "Curse you" or "Odin blesses you" or for a little more impact, if it's relevant "I would have said bless you but god would expect you to cover your mouth"
Mix up the gods you use for more fun.
Obviously some of these will get a reaction you may not want to deal with at the time so they have to be used appropriately.
I prefer invoking the names of "Dead" gods, such as Odin or Zeus, or Quetzalcoatl (Believe me, that last one is tricky). I like the looks people give me, because frankly it's just effing hilarious. We were having some sort of banquet of some kind for my graduating class or some shit like that, and the table next to mine gets really quiet, and I KNOW they're praying, especially since all at once they go "AMEN!" at the top of their voice. So I respond by hefting up my plastic cup of coca cola, standing, and saying as loud as I can "Fé, Vit, Friðr, Grið, Heill!" And drain the cup, then throw the cup in the dirt and shout "Praise be to Woden!"
Many weird looks were received that day. xD
That's GREAT. ( I'm a Zeus man, myself.