Post-it notes used as bookmarks are damaging: they can leave residue, tear pages, and even pull print off the page.
Paperclips are very handy but pages of books often show signs of their use -- crimped pages, tears, and rust marks. They should be used as little as possible if at all, and must be carefully removed when their function is complete. Paper clips should not be left on volumes to be placed on shelves for any length of time.
Avoid the use of rubber bands, if possible. They certainly should not be used for unbound materials going into any long-term processing location. Apart from other damage, as the rubber band rots over time its residue remains on the book.
Cotton tape, or string, are preferred.
