I just lay down and let my mind drift.
Some times when my body is exhausted and my mind isn't quite there I'll feel my body start to slip, and realize that my thoughts are becoming more erratic, and the images more vivid.
When this happens I usually start to feel an immense pressure on my chest as all breathing becomes completely mechanized and I can't control it myself, without waking up; and all my limbs become heavy and paralyzed. The shock of this almost invariably wakes me up, and I can't usually lucid-dream if I do reach sleep-paralysis.