"When you are in the middle of reading a book, and you need to stop, what do you use to hold your place? I shudder at some of the answers I have heard! The most outlandish one is a slab of raw bacon. Obviously, that should not be used for a host of reasons (no food products, please).
Another thing some people tend to do is turn the corner of the page over. Damaging the book is never a good idea. Other offenders include: hacksaw blade, pencils or pens, gum wrappers, and post-it note papers. The correct answer is a bookmark (preferably made from acid-free paper or card stock). If a bookmark is not acid-free (or you are uncertain), make sure you remove it (as well as any other papers) when you are finished with the book."
(from July's NYS Library Information Bulletin)
(Thanks to Jesse)