Dr Robert Cialdini came up with Cialdini's Principles of Influence: Reciprocity, commitment/consistency, consensus/social proof, authority, liking, scarcity, and unity. Using these are linked to increasing conversions.
These principles were put to the test in a study at Cornell University. Individual subjects were told they were going to evaluate art. Some of the subjects were given a bottle of Coke "bought" by an assistant who had slipped out to buy their own bottle. After, all the subjects were asked if they'd like to buy a raffle ticket. It was found that the subjects who received a Coke from the assistant bought two times as many raffle tickets as the ones who received nothing.