Hi,
My experience has shown me that any position and anytime works. However, if you're laying down to practice at bed time, I think you're training yourself to go to sleep. Lie down during the day if you want to practice that way, it'll probably help. Also, a heavy meal, alcohol, some other things, going to bed late, will most likely prevent a projection; because I believe it lowers your frequency. If you're already tired before you start, maybe some energy bouncing or chakra work would be more beneficial than trying to project; since you'll probably fall asleep because of the relaxation. A projection is still possible, but you may not remember it.
Grant