As a timesaver, I use Post-It Notes as bookmarks when I run games — specifically, the 1.5″x2″ size in 2 or 3 different colors (although any size will work). Before a session, I mark up everything I expect to need for the night — skill descriptions, creature stats, etc. — with different colors according to category: rules, descriptions, etc. I also stagger the Post-Its, so I can see all of them when the book is closed.

This lets me go straight to the info I need, and I never have to worry about bookmarks falling out. (I try not to leave them in too long, though, as they sometimes leave behind a sticky spot.)