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 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 tell them I no longer believe in that religious nonsense!

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

"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.

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

I generally don't go out of my way to tell them I'm an atheist. If they ask a direct question about my religion, I just tell them. It is obvious to anyone that has more than a surface relationship with me.

Many people derive great comfort from religion, which is fine unless they are trying to push it on me.

The best way is just tell them up front what you are. Saves a lot of time with closed minded people

It depends on the person. Sometimes I just say, no, I don't believe. If they want to continue with that, I give them as much as is appropriate. If they start to threaten me with eternal torture I say this in not so many words, by John Povlowitz:

"Do you believe in God?”
"People have asked me that question for my entire life.
The answer used to be simple and quick, almost involuntary. I had a tidy little collection of the platitudes and Bible verses I’d stockpiled, committed to memory, and carried around should I be asked. I’d learned it—and I could do it well.
But little by little, I gradually grew less comfortable with those easy answers and I had less and less peace in my spirit with what they implied. I look around at many of the Christians whose God I was expected to share and amen and defend—and I realize that I can not.
I listened to the celebrity evangelists and the partisan politicians and the brimstone street preachers, and knew that we were not speaking about the same thing. We couldn’t be.
As I read the Bible; as I reflected on the world I’d experienced and the people I’d encountered; as I watched what Christians were doing and saying in the name of God, I came to the conclusion that I had to make a distinction between their beliefs and mine—because the two were simply incompatible.
There is a God I do not believe in:
I do not believe in a God who is male and white. (though I will use masculine pronouns below, as this identity is critical to the beliefs I once had but have discarded.)
I do not believe in a God who created women as less-than; who assigns certain tasks to them, who ascribes different value to them, who reserves church and home leadership solely for men.
I do not believe in a God who doles out blessings like a cosmic Santa Claus; adding up our naughty and nice stuff, giving us good things if the scales tip in our favor and withholding them if we don’t measure up.
I do not believe in a God who answers prayers based on volume; who will move to bring healing and help—only if enough appeals are made, when a critical mass is reached.
I do not believe in a God who is capable of permanently writing off His children for their mistakes, their rebelliousness, their unbelief; who would craft a place of eternal torment and suffering and separation—and then send them there for good.
I do not believe in an all-powerful God, who would allow a devil dominion anywhere—let alone in the place where His supposedly treasured children spend their days, as hurting, vulnerable, and scared as they all are.
I do not believe in a God who commands me to forgive others unrelentingly—and then holds a grudge against me should I fail one too many times; a God who is as petty, judgmental, thin-skinned, and vain as I am.
I do not believe in a God who spoke to a handful of people a few thousands years ago through divine dictation—and who is now silent.
I do not believe in an all-knowing God, who would create men and women with a specific identity and natural inclination to love—only to find them repulsive as they lived into those deepest truths.
I do not believe in a God who would choose sides in any war; who would revel in violence, who would rejoice in death, who would celebrate genocide.
I do not believe in a God who blesses America—or any other nation.
I realize that to many Christians, this means that I am not a proper person; that my lack of faith is illegitimate, my lack of religion is heretical, my testimony nullified. I’m okay with that. I know that any bitterness or condemnation that they respond to these words with, is the voice in their head of the God they believe—and I understand. They are, trying to figure out what character is—and how we should live accordingly.
All any of us can do, is to be as honest as we can at any given moment, about where all our searching and studying and praying and living has led us. This is where I am. I can’t be anywhere else. Today when people ask me, “Do you believe in God?”, especially when Christians ask me—my reply isn’t quick or simple or nearly as tidy.
Now my response is, “How much time do you have?”

When some religious topic comes up, I say I don't believe in god anymore. I'm too old to have an imaginary friend.

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 don't. They start to figure it out as we talk.

Have you heard the fake news?

Mainly because the bornagains I’ve encountered ask if I’ve “heard the good news”... not sure what they mean, but they know what I mean...

"I am an atheist, it doesn't run my life and I am happy to discuss my beliefs with you"

Hhhhhh so hard to tell in Iraq

I live in sheltered accommodation North Coast of northern Ireland and most people here are religious. I try not to rattle any ones cage at all because I am different enough anyway but if I trust someone and they know I don't disrespect their choices I just say that I am an atheist. No one has ever questioned me about it or tried to change me in any way and I keep a neighbours statue of 'our lady' clean and tidy.

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."

It usually comes up when they invite me to church with them.
"Would you like to go to church with us?"
"No thanks, I'm an atheist"

I don't, I just play into what ever religious expectations they have. It's easier and I come to discover that spiritual choices are personal and should be treated as such.

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".

I read a great quote once.

Religion is like a penis.
It's a wonderful thing to have. I am quite proud of mine.

I am not going to force it down an unwilling persons throat or wave it in front of children.

You can let them have their invisible teddybear in the sky. It's not like there is some atheist god commanding you to convert people.

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 rarely have to. Aside from certain family members on holidays, I don’t find myself in the company of too many religious people that I am aware of. At least not outwardly religious to the point where they want to discuss it. But secretly I do like the occasional debate only because I always come out feeling like the winner of the argument. ????