I am myself rather fond of the symbols used in Avatar: the Last Airbender to depict the different elements (not to mention their nuanced and thoughtful approach to the elements in general):

I was also inspired by this article on Works of Literata about calling upon characters from Buffy the Vampire Slayer when calling the quarters (for me, it's Buffy = Earth, Willow = Fire, Xander = Water, Giles = Air).
How about you guys?