Shelf-markers: kids use them to keep track of what shelf they take a book from. I made these out of flexible plastic cutting boards (2 for a dollar at Ace Hardware!) each cutting board makes 4 shelf markers. I decorated them with stickers and numbered them in case I ever want to assign them. 
I also make all the kids keep their books in a book bag, which has greatly reduced loss and damage. I use these for grades K-3, and it's one of the best things I've ever done to keep things organized. The kids don't have to wait in line for checkout, they just fill their bags, and leave them with me. I do the checkout at my leisure (or have a volunteer do it for me, if I'm lucky). and deliver the bags to the classroom later in the day (or the next morning)
