I run into the problem regularly. Students have a hard time remembering where they pulled a book from if they decide they don’t want to check it out. This seems to happen more in the non-fiction section, where Dewey Decimal rules. We do our best, but the fact that it’s a challenge means more can be done. Like this for example:

Animal shaped shelf markers.

Click here to visit the website (in Japanese).

(Thanks to Gawker for the link)