Sit or stand in a non-slouched posture. Place one hand over the navel. With fingertips of the other hand, rub the space between the first and second ribs (just under the collarbones) on either side of the sternum.
Rationale: This perhaps works by the lower hand stimulating the vestibular system (via the solar plexus) and the upper hand stimulating the carotid baroreceptors (causing a mild parasympathetic shift)
Source: Dr Paul Dennison, described in the book Smartmoves by Carla Hannaford, Ph.D