It takes a lot of patience. Dont expect overnight results, like anything you have to work at it.
Saying that, and a lot of people will hate this or disbelieve me but I don't care- It took me my fourth attempt to be able to do it, but thats because I had been working on my chakras for about six and a half years prior to when I first attempted.
So work on your chakras, cleanse and spin them, study AP. My favourite books on the subject are 'Treatise on astral projection' by Robert Bruce and 'Astral projection for beginners by Edain McCoy.
Practise regular meditation, try lucid dreaming, mental projection and visualization exercises which include you walking around your room, and taking note of everything in there as if you are actually doing it physically, for example.
And take a relaxed attitude about it, dont get to over excited when you first leave your body as it may jarr you back into your body straight away. Dispel any fears about it, if you have any as that may also hinder the process. Study will help with that.
Goodluck, its the most liberating feeling to actually leave your body and you can learn many things from it.