the only 'technique' i know of is what rav3n said [expanded mind] you need to meditate, relax to the point of falling asleep. then there are many ways people can do it; float out of your body, fall out, even visualizing a rope or a bridge to pull or walk out. you should protect yourself just as a precaution mind, i've never heard of someone being possessed/attacked, but it never hurts to take precautions. the first time you do it, just float around the room, don't go any further, then go close to your body and think about returning. from what my friends who have AP and i have noticed, if you're far away from your body and think about returning, you'll be back instantly, but if you don't think of, say, returning to your room, then returning to your body, you kinda land with a jolt that takes a minute to get over.
AP is hard, my one friend did it once but was so startled by the feeling of floating he fell back into his body, and now he can't do it anymore because he was so freaked out the first time his mind/spirit won't let him. myself, i've read a few books and articles on AP but can only do it [even if i don't want to] on Samhain, and even then i just float around my room.
you have to have a deep need, and a purpose for projecting. if you just want to see what it's like, or you want to spy on someone, or you and your friend want to float around together, it probably won't work for you. [there was a line in a book on AP i liked 'you cannot use astral projection to be a psychic voyeur'.]