Though generally we don’t link to lists, this is fascinating. 50 Scientific Ways to be Persuasive, from the book of the same name. Here’s a couple standouts:

Giving away the product makes it less desirable. Researchers gave one group of people a picture of a pearl bracelet and asked to evaluate its desirability. Another group of people was given the same task, but prior to that was shown an ad, where the same bracelet was given away for free, if you bought a bottle of expensive liqueur. Second group considered the bracelet much less desirable, since mentally a lot of potential buyers (35% of them to be exact) shuffled the bracelet onto “trinkets they give away for free” shelf in their brain.

As time goes by, the value of a favor increases in the eyes of the favor-giver, and decreases in the eyes of the favor-receiver. Researchers asked a group of people in the random office environment to exchange favors and then rate the value of the given/received favor in their eyes. A few weeks later the same employees were reminded of the favor, and asked to evaluate the favor again. Favor-givers consistently assigned higher value to a given favor, while as the time passed by, favor-receivers tended to assign lower value to the received favor.

(via Kottke)