The best way to ensure prompt return of items is a huge cash deposit. My deposit is 1½ times the retail value of the item plus overnight shipping of said item (in case customer loses or damages the item).
I also have a detailed sheet they sign, describing usage and Ts & Cs, and a list of parts that need to be returned. They have to return it within five days of loaning/renting.
I deduct for late returns, dirty equipment, and lost or damaged equipment.