Summerbreeze also suggested to me to set my alarm a few hours earlier than usual. When it goes off, wake up a bit and then lie back down and begin your astral attempt.
When I followed this advice, it worked very well. And now when I go back to sleep in the mornings before the kids wake up, it will happen involuntarily sometimes.
Also, look around at the other threads about astral projection. You'll find some others like yours and there may be some good info to help you in them.