I have chosen to use a Pentacle.
The Five Pointed Star represent energy, fluid, breath, matter and mind. And the Circle around it represent a shield of protection.
It has a lot of history behind it. The Pythagoreans held the pentacle sacred to Hygeia, the Goddess of healing, whose name (HGIEiA) was an anagram for the elements water, earth, spirit, fire and air.
It also have a strong association to Satan and the devil when the star is pointing downward, as we used it on the logo.
(This comment references Entry #143)