As the supervisor of a "medium-rare" book collection, there's nothing more cringeworthy for me than to see one of our items with Post-It notes attached to the pages. While I love Post-Its as much as the next person, even the gentlest of adhesives can leave a chemical residue on the paper that can shave years off the life of a book when the glue breaks down (little critters enjoy eating many kinds of adhesives as well).
So what's the poor bibliophile to do? Use Book Darts! These thin metal clips will not only mark the page you want to remember, but since they're fashioned in the shape of an arrow you can actually use them to highlight the exact line of the text you're interested in as well. The brainchild of a former teacher, librarian, and archivist (as well as a boat builder, woodcutter, and printer!), Book Darts seem to be the perfect alternative to sticky Post-Its or who knows what other kind of horrors employed in the service of marking one's page.
I'm seriously thinking about buying a couple of boxes for the Reading Room and loaning them out as we do our bag weights.