I try to do something that symbolises the events of the chapter I'm drawing the cover of. So far, I've drawn, in turn, on my six covers: the main character's hand grasping the hilt of his sword, a bunch of writing tools surrounding another sword, a grinning demon, a bunch of secondary main characters against a rainy background, my main characters in a festival crowd, and a secondary main character dissolving into flames and ashes.
I plan my comic well ahead of time, and have roughly 2 chapters' worth of finished pages as well, so I even though I draw the cover right before I post it, I can look through the chapter itself and find something fitting to draw.
All the revisions and edits happen well before I have to draw the covers, because I'm so far ahead of my readers.