From the Inside Influence at Work Blog:
People are more willing to comply with requests (for favors, services, information, concessions,etc.) from those who have provided such things first.
People are more willing to be moved in a particular direction if they see it as consistent with an existing commitment.
People are more willing to follow the directions or recommendations of a communicator to whom they attribute relevant authority or expertise.
People are more willing to take a recommended action if they see evidence that many others, especially similar others, are taking it.
People find objects and opportunities more attractive to the degree that they are scarce, rare, or dwindling in availability.
People prefer to say yes to those they know, like and find attractive.