EDIT: i am sharing these notes, but they were originally created by ROB KOO at dreamworks' redwood studios...
a few years ago i tested for DW and they sent me these EXTREMELY helpful notes. some probably apply more specifically to animation storyboarding than comic panels, but a lot of it is good, commonsense advice for graphic storytelling in general.
although i am certainly not an expert on the subject, i strongly advise anyone interested in storytelling through comics or animation or live film to check out these blogs, and check them out hard:
there are of course many others, but i love these and think of them first. also, they have links to tons of other great blogs.