One of the most eye-opening books I’ve ever read is Robert Cialdini’s Influence: The Psychology of Persuasion. I read it several years ago, and it forever changed the way I interact with people who want something from me. His book has probably saved me thousands of dollars and hundreds of hours of wasted time.
Cialdini is a social psychologist who set out to understand the methods that everyone from marketers to con artists use to convince you to give them your money, your time, your loyalty, or other resources you possess, even when it’s not in your best interest to do so. Through extensive field research, Cialdini came up with six psychological principles that are used (consciously or not) by people who are in the business of trying to persuade other people to do something, often against their better judgment.
Trent Hamm of The Simple Dollar reviewed Influence and did a great job of summarizing the six principles of persuasion (they are: reciprocation, commitment and consistency, authority, social proof, scarcity, and liking). Even though I recommend that you read Cialdini’s book in it’s entirety, you’ll still get a lot of bang from your buck just by reading Hamm’s distillation of the book’s major concepts.