Here is another set of pages that you might want to print out and keep near, just like the ones in the post below: four great pages on Composition from Andrew Loomis's "Creative Illustration".
"Creative Illustration" may be the best book ever written on drawing and illustration. It's amazing, and can be acquired through ebay or a used book seller if you're so inclined. I think there were plans to reprint it recently but I think they fell through.
There's an even easier (and cheaper) way to get the book: if you type in "andrew loomis pdf" in Yahoo! or Google you'll be able to download pdfs of all of Loomis's books from any number of sites. All of his books are definitely worth checking out. "Creative Illustration" is particularly full of great information presented in a really clear and simple way; my selection of these four pages is pretty arbitrary - there's lots more great stuff in the book. I particularly like his page about the "fulcrum" idea - I only remember hearing that idea from one other book (Poore's "Pictorial Composition") and it was very confusingly presented in that book, I think.