I honestly don't usually have to. A lot of people around me are atheist or agnostic, or not religious enough to bring it up like...ever. I imagine a number of my acquaintances are Christian, but...they're acquaintances lol. With a couple exes, we've gotten to discussing religion and we have just said what we were. One was Christian, the other Muslim (by family, he only participated in fasting really).
The one good friend I made within the last 3/4 of a year just fairly recently (maybe three months ago, maybe four) revealed through casual conversation that he was Christian, and pretty religious. He doesn't wear a seat belt, and he says it's cuz he's got three protectors: His mother, his guardian angel and the Holy Ghost. Funnily enough, he's been in a decent number of serious crashes (as front seat passenger too) and been just fine.
Doesn't change the fact that he's really intelligent, ethical, kind, supportive and all that shit. He's even a little conservative in some ways and is still an amazing person. I believe the third person of that trio we hang out in is also agnostic or atheist, but neither of us have said so. The religious guy prolly has figured that one out (other man also may have told him at some point, they have a long-standing and very close friendship), but it really doesn't matter.
Anywho, sorry for the rant xD

"Oh, I'm not religious. Pray if you like, that doesn't bother me, it's just not my way."

Sometimes they'll inquire further, and assuming they have a reasonably friendly tone then I'll say, "When I said 'not religious' that meant 'atheist.' Some people freak out at that word, so I usually say 'not religious' instead. But it's an accurate way to describe me."

If they inquire further and aren't friendly about it, I say, "It's a free country and I'll respect your beliefs if you respect mine." Then I'll change the subject.

If asked or approached by theists I simply reply "I don't believe in fairy tales"

I tell them I no longer believe in that religious nonsense!

I simply tell them, “I don’t have a religion. I am atheistic.” in response, I usually get some version of, “oh, that’s OK, some day you’ll see the light.” I roll my eyes, or shake my head, tell them, “I already have found the light, that’s why I’m atheist.” That usually completes my contribution to the conversation, as they then usually spout off some version of Pascal’s Wager, and congratulate themselves for having the good sense to believe in fairy tales.

I find it better to not volunteer that information about myself. I wait to be asked about it. Somehow, they appear to feel threatened in some way by someone believing something other than they do.

I live in the South where religious people are the norm. I generally just say I'm not religious, because "atheist" is too much of a dirty word here. In really extreme cases I can get away with "non-denominational", that's the one I have to use with my family.

I am not an atheist, I am an agnostic (or a believer in my own set of believes). I explain that to them, treat them with respect and expect the same. If they try preaching on me I start playing the game and while listening to what they have to say, start preaching my believes to them myself. Some listen, some quickly give up. (And one made three cross-gestures against me and ran away screaming something incomprehensible).

My truck is covered with Atheist and Science bumper stickers so meeting new people wherever I drive is a done Atheism deal. ...if queried I reply EVERYONE IS BORN Atheist and should not learn violent religions anywhere on earth

I do not

I shoot straight. tell them I am atheist in fact I bring it up with out them asking if conversation leads to religion. it is up to them how they take it. won't hide my opinion as if I am living in the 15th century.

I don't bring it up. I even try avoid talking about it. I silently retreat to the background. They would not understand and it would screw their head up.

"I'm an athiest" ......." I only eat babies on the weekend when I have time to slow roast them"

I just wait for them to make some religious pronouncement at which point I tell them I don't believe in that deluded nonsense, Magical Easter Bunny In the Sky or bullshit depending on how inappropriate I find their pronouncement.

If someone asks me I simply say, "I am atheist." If I'm not asked, I have better things to do with my time than to bring it up and invite a possibly heated debate (yes, it makes some people angry.) However, if someone specifically asks me, I just put it out there: "I am atheist."

Have they asked you if you are atheist? If so, just say, yes. Otherwise, unless it comes up or is relevant, why would you just volunteer? Then If they respond, you can carry on with your end of the conversation. Even when I was a Christian many moons ago, I didn't just volunteer my religious views, unprompted.

When asked about my religion, I smile and very calmly say, "I don't believe in it."
Then, if they persist (in horror), "You don't believe in God?!?"
"No, I don't believe in any of it."
"What? Don't you believe the Bible was written by God?"
"No, I don't believe in it."
"Well then, what DO you believe?"
"I believe I'll have another beer. What about you?"

I have found it very uncomfortable and haven't even tried to tell a few devout christian and muslim friends. I think they would be horrified and feel I had tricked them But I cannot stand listening to them spout their prayers for me and other blathering comments, I go into a coma and grind my teeth. I wish I could say something, but I'm SO uncomfortable. Any realistic suggestions ?

I don't mention it, if they mention religion I just tell them they are talking shit. I'm pretty direct, really. But it's certainly not enough for two people to not get along, certainly not in England!

My response is just simple and nonchalant. If they ask me about my faith I just tell them I have none, I'm atheist. Tell them I'm humanist in terms of world view, but only if they mistake atheism as a world view. Canada isn't as crazy as other parts of the world, I still get the odd funny look, but most don't care that much.

Just say something like "I don't believe it" and let it go at that.

Sometimes it is easy to drop into conversation, sometimes not.

Sometimes I wear a T-shirt or jewelry that out me before I open my mouth.

I am at that point in my life that I don't care how believers view me. I am thus quite open with a bit of my sarcasm included. I tell them that I have matured past the point of believing in imaginary characters.

I usually just say I’m not religious. If they get pushy I say I’m atheist, living in California it’s rare that people get pushy.

I wouldnt say anything if they are interested in the reason you don't go to church you could tell them then. Christians arent stupid they just believe in something without proof. Some might even become athiests themselves.