Well it's more of a muscle trick. You can do it this way:
Close your eyes.
Open your mouth.
Imagine a cord that's holding your shoulders up gets cut and
they fall down. Sometimes they'll go up again, but then you have
to cut another cord until they stay down.
Slow your breathing.
When you're feeling sleepy a tiny bit, open your eyes and close
your mouth and breath through your nose slowly.
That's probably your neutral face. It leaves you with the
feeling that your face is melting a little, at least it does to
me when I do it.
