Here is an interesting article in the Times about how subtle changes in cafeteria serving lines can influence item selection by students. This is a great example of how designers can manipulate consumers in positive ways by studying their response to specific stimuli.  By carrying out experiments, it is possible to know for sure what works.  This is design research in action, and it is quite easy to do: you just have to change one variable at a time, and see what changes. If you make all of the modifications at once, it becomes much harder to correlate causes and effects. But if you do it in a methodical way, you can be very confident that you know which change led to which result. 

Here's a link to the article in New York Times. The illustration showing all of the different design ideas is excellent (as with many graphics they do), and it has some nice meta data that you access by hovering the mouse over the little letter i's.