first, wrong category, should be astral projection.
i don't know of one to prevent someone from projecting [kind of mean in my opinion] but i think i might of seen something once. what i do know is if you have a strong enough shock to your system you'll stop projecting for a while mostly out of fear. got a friend who suffered a really nasty 'crash landing' a few years back, hasn't projected since.
there are books and people who know more than i, but basically, you need to meditate until you're so relaxed you're about to fall asleep. from here, there's many methods to project, the most common [i think] is visualize a rope and pull yourself out. you can also try falling out of your body, floating out of your body, standing up, rolling, visualizing a bridge, and many many more, it's personal really. the first couple times you project i'd just float around your room to get the feeling of things. when you wish to return to your body, think of your body and you'll wake up. to get to the astral, think of the astral [try visualizing it a little, if you don't know where you're going, think of somewhere in the astral you'd like to be. myself i always go to the den of the dragons, so i think of that, but you could think of a field or water.] when you wish to return home, think of your room, then your body. thinking of your body will zap you back in with a jolt.
some things i've been told, don't eat before projecting, if you've eaten a heavy meal it'll be harder for you to project. if you decide to lie in bed, don't put covers over you [a light sheet is fine but no quilts] for the same reason. look for some beginner books, and some websites, for more detail.
here's a basic exercise everyone i know who projects does: http://www.wikihow.com/Perform-Astral-Projection