I remember reading this in Freakonomics but with a bit more embellishment. I remember that when the payment rates started to sag, he would put a picture of eyes by the money box, and that often helped the rate rise again.

A bored office worker could do a bit of his own experimentation in this vein -- how many donuts do people buy when there's a fake rubber cockroach near the box? A picture of a severely obese person, or a very fit person, or a salad?