Just chiming in to this thread. In addition to some of the other aforementioned ways of getting overdues back (emails, reminder notes home, etc.), one thing I've tried in desperation that seems to work (note: I am at the elementary/middle school level) is printable wristbands. You can buy them for inkjet printers and print out a generic message on them (i.e. "Remember to bring your book back!") or take a sharpie and write the book title on a wristband and then just put in on your student's wrist/backpack as a reminder. Sort of like an updated "tie a string around your finger" reminder from days of old. 

Not sure if this is something you'd like to try or not, but I use it sparingly (and only in desperation) and it often brings results. (Photo attached)

Kristen Frost

St. Bernard School, Enfield