1. Get into a comfortable position.
Obviously, if you're in your bed, this is a non-issue. But if you're out and about, get into the most comfortable position that's feasible (i.e., lean your seat back if you're in your car; find the most comfortable chair in the conference room if you're napping at work).
2. Relax your face.
This is key to the whole thing. You have 43 muscles in your face, and they're a big part of how your body knows whether you're stressed. When you fully relax your face, you send a physiological signal to your body that all is well. It's safe to sleep.
So close your eyes and relax your whole face: forehead, cheeks, tongue, and jaw. Let it all go slack. You'll notice as you do this that your breathing naturally starts to deepen and slow.
Now make sure your eyes are fully relaxed. You have six muscles in your eye sockets; feel them all go limp.
3. Drop your shoulders.
Let them get heavy, and then let them go completely, as if they were falling down toward your feet. Let the back of your neck relax and go limp. Take a deep breath and let it out slowly, releasing any remaining tension there (most people store most tension in their shoulders, necks, and jaws).
Now your arms: Feel them get heavy and relax, starting with your dominant side. If you're right-handed, start with your right bicep and feel it relax. If it's not, tense it fully and then let it go slack. Repeat the process with your hands.
4. Let your legs go limp.
Feel your right quad sinking down, getting heavier and heavier. Next your right calf, ankle, and foot. Repeat on the other side.
5. Clear your mind for 10 seconds.
Now that you've fully relaxed your body, all it takes to fall into a deep sleep is to turn your brain off. (This is like that moment after you switch your iPhone off when it takes another few seconds for it to fully power down.)
What you really want to avoid are any thoughts that involve movement ("I've got to pick up that drycleaning tomorrow"; "Did I remember to put out the recycling?"). These thoughts actually prompt involuntary movement in your body. You don't realize it, but just thinking about something causes micro-contractions in certain muscles.