In the book Exploring the World of Lucid Dreaming, there is a WILD technique called the No Body Technique. It doesn't involve concentrating on any imagery or sounds, but includes a gauge for when to let go that isn't insomnia or sleep paralysis.

THE NO BODY TECHNIQUE

1.Relax Completely
After awaking from a dream, lie on your back or right side with your eyes gently closed. Tighten and then relax your face and head, neck, back, arms, and legs. Completely let go of all muscular and mental tension, and breathe slowly and calmly. Enjoy the feeling of relaxation and affirm your intention to consciously enter the dream state; let go of all other thoughts, worries, and concerns. If you have just awakened from sleep, you are probably sufficiently relaxed. Otherwise, you may use either the progressive relaxation exercise (page 53) or the 61-point relaxation exercise (page 54). Let everything wind down, slower and slower, more and more relaxed, until your mind becomes as serene as the calmest sea.

2. Think that you will soon no longer feel your body.
While falling asleep, concentrate on the thought that when you fall asleep your body will become imperceptible.

3. Float freely about the dream as an ego-point
As soon as you can no longer feel your body, imagine that you are a point of awareness from which you perceive, feel, think, and act in the dream world. Freely float about the dream world like a mote upon a sunbeam.

Commentary
Some people will probably feel that life as a disembodied spark leaves something to be desired. If so, never fear, there are plenty of vacant dream bodies available for immediate occupancy.