Ahem. Firstly I would like to say this:

Please do not suffocate yourself. That is bad.

Nextly, have you previously experienced sleep paralysis? It can be pretty scary. It happens to me more often now. I would say that if you want to trick your body into falling asleep while your mind remains conscious, this technique has worked best for that:
Lay perfectly still, not moving a muscle (swallowing is okay, and please - keep breathing). Count in your head, "1, I am dreaming, 2 I will remember my dreams, 3 I am dreaming, 4, etc." I usually reset at 100. You can change your mantra to whatever you want it to be, but the point is that it keeps your mind active and you can actually feel your limbs/body falling asleep, usually starting at your feet and moving upwards.

Another similar technique is the 61 point technique.



I'm taking this excerpt from Stephen Laberge, The World of Lucid Dreaming (1990).

1. Study the figure

The figure below illustrates 61 points on the body. To do this exercise, you need to memorize the sequence of points. (This is not difficult, because the points are arranged in a simple pattern.) They begin at the forehead, travel down and up your right arm, then across to your left arm, down your torso, down and up your right and left legs, then back up your torso to the forehead.

2. Focus your attention on one point at a time

Begin at your forehead. Focus your attention between your eyebrows and think of the number one. Keep your attention fixed at Point 1 for several seconds until you feel that your awareness of the location is clear and distinct. Think of your self being located at this point. Before moving on to the next point, you should feel a sense of warmth and heaviness at this spot.

3. Move through each point in sequence

In the same manner, successively focus your attention on each of the first thirty-one points. Proceed slowly, and imagine your self being located at each point as you reach it. Feel the sense of warmth and heaviness before moving on. Do not allow your mind to wander. At first you may find this difficult to do; you will discover that at times you suddenly will forget that you are doing the exercise and start daydreaming or thinking about something else. If you lose your place, return to the beginning or the last numbered point you attended to, and continue. Practice with thirty-one points until you can attend to them all in sequence without daydreaming or losing track.

4. Extend your practice to include all sixty-one points

When you can attend to thirty-one points in sequence, repeat Steps 1 and 2 with all sixty-one points. Practice this until you can do all points without losing your focus. Now you are ready to use this exercise with lucid dream induction techniques.
Good luck!