Six tools for influence:
Reciprocity - People tend to return a favor, thus the pervasiveness of free samples in marketing. In his conferences, he often uses the example of Ethiopia providing thousands of dollars in humanitarian aid to Mexico just after the 1985 earthquake, despite Ethiopia suffering from a crippling famine and civil war at the time. Ethiopia had been reciprocating for the diplomatic support Mexico provided when Italy invaded Ethiopia in 1935. The good cop/bad cop strategy is also based on this principle.
Commitment and Consistency - If people commit, orally or in writing, to an idea or goal, they are more likely to honor that commitment because of establishing that idea or goal as being congruent with their self image. 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. Cialdini notes Chinese brainwashing on American prisoners of war to rewrite their self image and gain automatic unenforced compliance.
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 and the My Lai massacre.
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.
Source-- Robert Cialdini - Wikipedia, the free encyclopedia