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