It's so simple that you almost can't believe how powerful it could be. Basically, the researchers painted a path, complete with bright yellow footprints, from the latrines to the hand washing point. This was painted bright blue with yellow hand prints and had been raised on top of a concrete block to be the right height to make it easy for the primary school children (in Bangladesh) to use.
Then they made sure there was soap available, and sat back and watched what happened. There were no lessons or reminders to the children of the importance of washing their hands. And the results were amazing! You can see, from the graphic below, that just 4% of children were observed washing their hands with soap before the experiment. But two weeks after painting, 74% of children were washing their hands with soap - and six weeks later, that figure remained true. The study was carried out by the University of Oklahoma's Centre for Applied Social Research, with Save the Children and the University of Buffalo. You can read about it here.