So if I were taking a survey on proper and common bookmarks and asked the question: What do you use as a bookmark? What kind of answers do you think I would hear?
Please don’t tell me you bend a corner of the page or lay the book face down, which is bad for your book because it breaks the spine. Anything and all other items are acceptable and some probably not pictured here, like a paperclip.

Shoelace
Q tip
Ruler
Coupon