Animator John Kricfalusi has written a neat post on composition, using examples from Yogi Bear and N.C. Wyeth (!). All I have to add is that these are exactly the things I think about when I draw a picture. Also, as John K admits of himself, I struggle with it. This whole idea of seeing a drawing as blocks of elements and negative space that can be composed to steer the viewer's eye and advance the action is both important and difficult. It's another one of those situations where the easier it looks, the harder it is.
In addition to that, I work hard to combine an awareness of composition with the practice of spotting blacks--that is, placing large or small areas of black to emphasize or de-emphasize what you want and lead readers around by the hand without them realizing it. I feel I'm still low on the black-spotting learning curve. Then you have to think about not just the composition within a panel, but the composition of the panels and spotted blacks within the entire page (and sometimes in relation to the facing page). Then on top of all that, you've still got the basic requirement of clearly staging the characters and telling your story. It's a lot to think about.
Anyway, John K's post is a good, quick read if you're interested in that stuff. Thanks to Heidi MacDonald for the link.