One thing you should do when using disposable wedding cameras at your wedding is have a drop off box at the exit at your wedding. This way, guests wont forget to give them back or accidentally take them home. Also, if they see the boxes as they are moving around the reception hall they will remember to be using them.
Finally, I think its a good idea to tell your entertainment person to announce multiple times throughout the evening that guests should use the cameras!!
Thats all for now,
-Deb