- Timing is everything. If you're financially dependent on devoutly religious family members, i.e. if you're a minor or financially unstable adult living in their house, now is probably not the time. I've heard many accounts of teenagers being kicked out of their parents' house for leaving the parents' religion, and the reaction to atheism is especially harsh. After all, we're trusted less than rapists. So, you live with highly religious parents who you think may kick you out, wait until you can move out to tell them. For everyone else, it's best to pick a "neutral" time. You don't want to tell a religious friend who's going through a break up that they have to worry about your impending damnation, nor do you want to bring down a fun day at the park with serious talk (even though it might seem sensible to tell someone while they're in a good mood). Tell them during a more casual outing, preferably in a more relaxed environment. This brings me to the next point...
- It's not actually a big deal, so don't make it a big deal. This might seem counter-intuitive, but it's the truth. No one gets uptight about telling their friends they don't like rock climbing, or that they don't believe heavy metal music is any good, so why is this a big deal? The truth is, it's only an issue because we, as a society, make it an issue. There aren't really many differences between most atheist and mainstream religious people in the western world, despite public perceptions. Calling someone to meet with you to share a "big announcement" with them (as some atheists I've met have done) can add unnecessary tension by building up an expectation in the mind of your friend/relative. They expect to be shocked by this announcement, so it will shock them.
- But don't be too casual. Being an atheist is not everything to anyone, but it can be an important detail for some people. For people that you're certain will take it well (other atheists, people who are apathetic to religion, very moderately religious people) simply saying "oh, by the way, I'm an atheist" should be fine. For everyone else, the announcement should be made with at least a degree of formality.
- Know what you're going to say ahead of time. You don't need to script everything you'll say, but you should definitely have your thoughts collected. You don't want to be stumbling over words, nor do you want to say anything that might be interpreted as offensive.
- Understand their concerns. People who take their religion somewhat seriously will have concerns, and it's important to realize that they actually are valid, to them. If their religion is true, as they believe it to be, you may face real severe consequences for your unbelief. They care for you, and do not want anything bad to happen to you. They may also take it as a personal offense, just like many political ideologues take offense at the rejection of their ideology. While you do not want to get into an argument with someone who's in a potentially heightened emotional state, it's important to stress that your rejection of your old belief system, their religion, is at least as valid as their belief in it. It may not be truly satisfactory, but it is as close to a good answer to this. They may also have concerns for you that are not based in religion. Any major change in belief system or lifestyle can be a cause for worry for a variety of reasons, and they may feel that this is a symptom of a bigger problem. Of course, the stereotypes that we're a depressed, nihilistic group or that personal tragedy is the main cause of atheism in most people only helps this assumption, so it's important to assure them you're OK and that your atheism was not something that happened overnight.
- Don't make it too much of a surprise. This is something that I did tell TCC, as he said here. It's something I did. I'd drop hints at my atheism to my friends and family whenever possible. I'd criticize religious leaders, I'd attack anti-scientific religious beliefs such as creationism, I'd attack the theocratic positions of prominent right-wing politicians, and occasionally I'd even attack the notion of faith itself. This tactic has a few advantages. Firstly, one can be against theocracy, creationism, etc. without being an atheist, so you aren't going against point #3. Secondly, you're being honest, and so you can rightly claim you weren't deceiving anyone when the news finally breaks. Thirdly, and most importantly, by dropping hints at your atheism you may force them to privately come to grips with the fact that you may be an atheist, which can make their reaction to the news much better. They may even ask you about it outright, which would of course take the pressure off you. Granted, this could backfire, as someone you do this to may gossip about you and "out" you before you're totally ready, however, you can counteract this by not associating with total shitbags.
- Be prepared to answer questions. This should almost go without saying. Your friends and relatives will wonder why you've left the faith, and on top of that, you might be the first admitted atheist they've ever talked to. Be prepared to answer the question of "why", definitely, and probably the question of when it happened as well. You may be asked some other specific questions as well, and you may even be asked questions designed to lead into a debate. Do not take the bait. This is important. Answer the questions, but if they try to lock you into a sort of debate let them know, as politely as possible, that a debate is not a good idea at this point.
- Be wary of potential consequences. This is a big one. Even though this is not a really substantial issue, many people think it is. Unless you're employed by a church you can't be fired for your religious status, legally, but if you are fired for that, the legal fees for fighting it could be cost-prohibitive. Even if you aren't let go for that explicitly, it could be the real reason behind your termination. Atheism can, and probably will, cost you friendships or other relationships, even if you're not a dick about it. I've lost several friends for my atheism and I know of a few marriages that ended due to one partner losing faith. In some areas, being atheist can make you a social pariah, or worse. You need to be aware of this, and decide just how "out" you want to be. Do you just want your close friends and family to know? Do you want to keep some friends or family members in the dark? I know I did, I kept my atheism from my now-deceased grandmother because I knew it would have destroyed her. Are you confident enough to be fully out, or even become a non-anonymous blogger, advocate or activist? These are important questions you must answer, and it's even more important to not judge others for their decisions. As much as we need more open atheists, not everyone is in a position to come out.
That's about all the advice I can think of right now. If I can think of anything else, or if anyone else can, I may add it to this post or make a sequel post. Hopefully, this will help somebody.