I'm normally loath to throw up youtube recommendations but there is one person who is as good as any on the subject - certainly more comprehensive than any other there.
He presents several techniques. If you're already aware of them, then please skip the rest - about preparation; ways to start and so on. I learned about astral projection some while ago but an acquaintance recently wanted to know and it became apparent that an "external" person (someone outside her circle of friends) may be better so we found this guy.
He talks about getting in a hypnogogic state (which in itself is quite an art), then ways to lift or hoist your way out of your body. He stresses the need for body awareness in an 'astral' state - just as you're aware of your body in dreams, even if you aren't lucid. (I mean, you dream in the first person (obviously) and it's this kind of feel you're aiming for at first, to get to be aware of your astral body.
You may not agree with all he has to say, and this may not be for you but if you are having difficulty exiting body, it's worth the 10 minutes or so. And it could save you wading through masses of book stuff.
http://www.youtube.com/watch?v=ojsS35VQ2Y8
There's also this little item that might help.
http://www.wikihow.com/Perform-Astral-Projection
It'll take time. Takes a lot of patience. You may need to set aside half an hour, an hour even, at first. It's important to practice regularly - and to experiment with times of day - some times are easier than others. When I first started with this I found it a lot easier on first waking up, before getting out of bed so I set an alarm half an hour earlier than previously.
Lastly, don't forget the affirmations - that you are going to do it!
cheers,
L