Assuming that it came up (because I wouldn't bring it up), I usually say "I'm not a believer". This way I didn't say there is or isn't god. I also haven't said that I believe there isn't a god. Which is true; I cannot prove the negative, yadda yadda. I simply say that I am not a believer.
I'll listen to them for a bit, if it goes there, but then I usually try to terminate the discussion. It is rare that I have a mature and intelligent conversation about it anyway.
I do believe that my religion shows in my behavior. Judge me by that.

I usually tell them that now that I am grown up i tend not to believe fantasy stories as being real life. When they ask why I don't believe in god/jesus etc I let them know that I stopped having imaginary friends when I was 6 Y/O.
As johnnyfdny says it is difficult to to have a mature conversation with most religionists ....did I just create a word? It's true...I am a Mangod!..see out of nothing I just created something..

Generally speaking, in my country (the UK) no-one speaks about religion, unless they are evangelizing and trying to recruit for their (usually failing) church. I have lived in this small West-country market town for 30 years, and I have been approached just once by young keen and fresh-faced people about joining their church. When I showed them my ordination card for the CFSM, they were quite happy to sit down and talk about it, and would even acknowledge that there is a lot wrong with the Bible, and when I pointed out the advantages of the CFSM, one acknowledged that he'd heard of it and found it 'interesting'. When I said that of course it was a spoof church and that I was an atheist at heart, and had been for 40 years, they lost heart and went to pester someone else.

You say, “I am an Atheist.”

The subject rarely comes up, but I generally just tell them flat out. Which inevitable leads to, "I'll pray for you." I always reply, "I'll write a letter to Santa for you." They tend to shut up.

I tell them I’m an atheist. If they cannot accept that simple statement, well, then the conversation comes to a quick end. I don’t fault them for their beliefs, as long as they accept that having differing opinions does not make someone a “bad” person. Just, please don’t tell me that “some day you’ll see the light.” I do not express condescending attitudes towards theists, and I require the same consideration from them.

I just tell them I'm not religious

Yes there is a better way. Educate yourself in geology, cosmology & physics, and talk to them. Don't be in any rush to convert people. Just help them to awaken the rational part of their brain, if you can. Worldviews don't collapse in a single moment.

I may actually have enough points to comment! This question really resonated with me because I have recently become immeshed in a complicated situation with a friend who has just re-discovered God. She is giving all her power to Him, claiming no power or responsibility for anything she says or does. It's all in God's hands and if He wants me to . . . iI's a long story that I don't want to get into if my comment isn't actually accepted. I'll pull a Great Orange One by ending this part of my comment with . . .

At work, I don't have to tell anyone. I've told a few people. Now they talk about it when I'm not around. Here in Kansas, Christianity is the extreme default. So anything else is to be pitied. But I did have some ladies come to my home once with flyers. I told them I don't believe. They both said "ohhh" like my dog died.

I just say it or that I do not believe in a supernatural magical being in the sky that I have to have blind faith in BEFORE it will help me out here on earth. All the religions I have practiced, including a sect of buddhaism, insist I have blind faith & belief before the "god" or "universe" shows benefit in my life. Bunch of hooey.

I am up front and let them know my position early in conversations... I work with staff and clients who come from many religious positions ... so my approach to remain authentic to my self is to say "I am a person of no faith... but respect your choice to choose what you believe in. I have thought about using the word "Atheist" but find 'a person with no "faith" softer and less confrontational as I work in a health context.. ?

I tend to allow conversation to be the light in the tunnel. At some point I will mention I'm a raging atheist.

I live in a very rural fundamentalist Baptist area and I live alone on a farm so the one thing I do not do here is tell people my true beliefs because I can about guarantee my place will be vandalized if I do.

I am not an atheist I'm not agnostic I'm an Orthodox Jew

I wait til they ask me or make some comment - I told my right wing religious mother that I will never go into a nursing home, that I will hasten my death if I can't take care of myself. She said "If you do that you will not be allowed into heaven." I replied "I don't believe in heaven or hell. I am atheist." She was very surprised but did not cry or get angry. At 87 years old, she has become a bit more mellow.

I have a problem with the word "Atheist". It is negative and too many religionist feel it is an 'in your face' term (like many of theirs). Languages change and often bring about a change in attitude. Look at the change in how people view the gays.

There are positive ways of expressing oneself. Instead of godless how about godfree? (same for childless - free). Not many people know of the "Brights" movement. It is an international, on-line group (my late partner - wife has too many possessive connotations - once did a Farsi (Iranian) review for them). A Bright (noun) is one who has a naturalistic world view; a 'super' has a supernatural world view. Look them up, there is a big following.

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 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 don't go for shock factor but there is a certain satisfaction in waiting until a person knows you before you tell them. The first response I usually get is, "You seem like such a nice lady!" and I reply, "I AM a nice lady." Most of my family and friends know I'm atheist. Many of my co-workers do also. I don't like to lie about who I am.

Well, I'm agnostic and I just tell them that I'm not religious. Even if I live in the Bible belt, until now it never gave any problems. Just an understanding okay, or something like that.

There is little reason for wearing your religious belief on your sleeve. Atheism doesn't require you to go out in the world and create converts. If the subject should come up, just say, "I am an atheist." If they try to persuade you to change. Let them know that you expect them to respect your coice, as well as you respect their choice. If they persist, fart a few times and leave.

What I say sometimes depends on the audience I am speaking to. Try saying, "I'm non theistic". If they enquiry further, say, "I believe in science, ethics and the immutable laws of physics". If they still enquire, say "I think of magic as entertainment".

I do not discuss religion or other choioces with people. They should not ask. If they have the balls, then I just say happily I go to my church, mother nature, and bless her everyday.

Everyone I know is already aware that I am an atheist. I've had many conversations though, while having a drink at the bar with people who inevitably, bring up their religion in one way or another and at that point I let them know that I'm an atheist. Probably why I can't find a date.