From the Harvard Business Review:
When we consider whether to do something, we subconsciously ask ourselves a simple question: "Am I the kind of person who...?" And money changes the question. When the lawyers were offered $30 an hour their question was "Am I the kind of person who works for $30 an hour?" The answer was clearly no. But when they were asked to do it as a favor? Their new question was "Am I the kind of person who helps people in need?" And then their answer was yes.
People tend to think of themselves as stories. When you interact with someone, you're playing a role in her story. And whatever you do, or whatever she does, or whatever you want her to do, needs to fit into that story in some satisfying way.
When you want something from someone, ask yourself what story that person is trying to tell about himself, and then make sure that your role and actions are enhancing that story in the right way.
Read the full article here, it is well worth your time.