How do you get more people to use the stairs rather than cram onto a narrow escalator? This approach worked in a Swedish subway station:
Or how about recycling?
Link to stairs video: http://youtu.be/2lXh2n0aPyw
Link to the recycling video: http://youtu.be/zSiHjMU-MUo
Link to the Fun Theory folks: http://www.thefuntheory.com
(More videos at the latter link.)