Robert Cialdini has written the interesting book, Influence: The Psychology of Persuasion.

Cialdini explains, with several examples, the techniques of persuasion that make people conform or act in a certain way. As the author states, we are overhelmed by information and by the fast pace of modern times. When making a decision, we will less frequently enjoy the luxury of a full analysis and we revert increasingly on shortcuts. There is nothing wrong with the shortcut approach, the problem comes when the normally trustworthy cues are created to lead us to erroneous actions. Therefore, it is important to recognize exploiters and avoid their attemps to make profits on our shortcuts.

Also Guy Kawasaki is a fan of Cialdini and he suggests other theories as well such as power, friendships, and emotion. Read more at his blog: http://blog.guykawasaki.com/2008/07/how-to-change-s.html

Cialdini six “weapons of influence”:

• Reciprocation – People tend to return a favor. Thus, the pervasiveness of free samples in marketing. 

• Commitment and Consistency – If people commit, orally or in writing, to an idea or goal, they are more likely to honor that commitment. Even if the original incentive or motivation is removed after they have already agreed, they will continue to honor the agreement. For example, in car sales, suddenly raising the price at the last moment works because the buyer has already decided to buy. 
• Social Proof – People will do things that they see other people are doing. For example, in one experiment, one or more confederates would look up into the sky; bystanders would then look up into the sky to see what they were seeing. At one point this experiment aborted, as so many people were looking up that they stopped traffic. 
• Authority – People will tend to obey authority figures, even if they are asked to perform objectionable acts. Cialdini cites incidents, such as the Milgram experiments in the early 1960s.
• Liking – People are easily persuaded by other people that they like. Cialdini cites the marketing of Tupperware in what might now be called viral marketing. People were more likely to buy if they liked the person selling it to them. Some of the many biases favoring more attractive people are discussed.
• Scarcity – Perceived scarcity will generate demand. For example, saying offers are available for a “limited time only” encourages sales.