I do not

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

You can ask them

why would you be talking to one anyway?

I usually say " I'm not a believer" , but only when asked. If they can't leave it at that, then they get the "superstitious nonsense" reply.

I only bring it up if it's brought up and if it is my usual statement is, "I don't buy into what I think is a bunch of bullshit, sorry."

You can say, "I find no reason or proof to believe that way. What is your idea of "god" & why do you believe?" I, myself, do state that I am an atheist, but I am trying to be somewhat less confrontational & attempting to master some of the lessons of "Street Epistemology".

Very easily, I tell them as soon as they ask. I don't offer the information that I am atheist until they ask though.
Generally it is unproductive to engage the religious in conversation about religion. You certainly won't change their minds, and in fact you will probably drive them further down the path of faith if you attack their faith.

I try to do it in a humorous way as follows. ""If Joseph and Mary were alive today and Mary were pregnant, then Joseph would be getting Mary on the Jeremy Kyle show to do a DNA test".

I'll get some shock looks but I'll just say to them that I'm sorry, but I don't believe in immaculate conception. If that isn't enough of a hint I'll just straight out say I'm atheist. At this point some people turn into snowflakes and get offended.

I live in Waco. I just stay away from them as i burst out laughing at things like signs that say, "pray away the gay", these idiots believe that stuff. The religious people here are all hypocrites and it's quite disgusting. They think God likes them better and rewards them with money. If I tell them I don't believe they want to talk to me. No, oh, no.

Providing Atheism, or religion is the conversational topic I simply make it perfectly clear that I am an Atheist. To the religious fanatics who push their advice onto me as to what is going to happen to me because I'm an Atheist I simply reply it is because of you and the way you treat others who don't follow your rules that convinces me to remain where I am.
I do not celebrate the festive season because of the hypocrisy which surrounds it. Watching people gorge themselves surrounded with cheap and trivial rubbish representing their particular faith makes me wonder about humanity. Half the world is starving while the other half are on diets. Amongst the social community in which I live those who profess to be connected to one faith or another, there is not a single person who has visited a church or place of prayer in the past two or three years. Any attempt to discuss and come to an understanding of their behaviour is generally met with a strange look implying that I must be the strange one because I'm not following the herd, followed by the justification that '...anyway it's for the kids! ...'. I wonder just what are we teaching them ?

That I can well-tell where religions, gods came from... there are remnants, vestiges of their origins in various shamanic, animistic societies to date. That such belief and conviction systems likely devolved to explain scary natural phenomena... lightning, death... And to, yes, Control other people's lives and assets. That such systems have no attraction for me; in fact, I consider them a vexation of "the spirit", and encourage them to consider living their lives apart from such superstition.

I don't.
In the same way that I don't tell people I am heterosexual. 99% of the time it isn't relevant to our discussions. Should the conversation turn to religion, then I am comfortable in stating that I am an atheist - and then often then have to explain the difference between atheist and anti-theist as most religious people seem to think that if you don't believe there is enough evidence to believe there is a god, then you must believe that a god cannot exist.

I don't unless asked.

I don't tell them.

I live in Texas so I don't broadcast my atheism but I will fess up when asked.

i agree with people saying that you should hold off on telling people. let them get to know u before revealing this to them. also maybe even getting to know them and seeing if they seem mature enought to handle it would be good too. or hell sometimes even just keeping to yourself may be a better option

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.

While rangering in the Uintas, I spoke to a backpacker who was obviously just off a LDS mission. He began his religious line, then I told him I was LDH; Church of Jesus H. Christ of Latter-day Hedonists. That shut him up, and I was able to squeeze in a little bit about "Leave No Trace" camping.

I usually don't. I pick apart their logically fallacious arguments while letting them assume I'm just a bad Christian. Otherwise there is no talking to theists, or least no listening from them.

I just say that I am an atheist

there is nothing more difficult than saying that in Arabian world and I was saying that. but first you have to know the person and show him that you are a good person.

Usually only if they ask me directly.

Here in Arkansas, I just keep my mouth shut.... I don't want to talk about something that'll just make no difference, and have people "pray" for me.... I don't have time for it.

Generally, other peoples beliefs are not important to me and don't impact my life. Their religion is none of my business as mine is not theirs. I do not go out of my way to tell someone I'm an atheist but if pressed I just say their beliefs are not mine or their god is not mine.