Sorry, I didn't clarify... you need the lucid dream state in order to actually leave the dream, because the astral level relies on the same kind of unconsciousness state as dreaming. Some people just skip the dream and go right to the astral, but for people who don't do it naturally, using the dream state is the best approach. The only model I have found that sort of explains it is the theosophic model.
There are many levels to consciousness in this system, but the only ones you need concern yourself with for understanding AP are these: physical, etheric, astral, mental, causal.
Both dreaming and AP take place on the astral level, but the dream state is a lower vibration and more linked to the emotions. People who have a hard time getting control of their dreams have a lot of unresolved emotion turmoil usually. In order to leave your body, you have to raise the vibration, which involves moving beyond the dream state, which involves not being attached to whatever is going on in the dream.
A good example is that recently I was in a dream where I was coming home from a party, and the night sky looked incredibly cosmic and full of stars. In the dream I looked up at the stars and started to rise up, flying toward them gradually. As I did this, I could feel my body tingling, and I realized it was a dream. I broke through and then left my body, standing in the middle of my bedroom.
I've done it by accident too. I've got up in the middle of the night to use the bathroom and fell through the toilet because I didn't go with my body. One time I was napping and the post man knocked at the door, so I went to the door to open it but my hand went through the doorknob. Once I realized I flew back into my body with a whoosh that scared at the hell out of me.
Anyway... just experiment. Setting intentions around bedtime is important.