As I said, clinical psychologists have studied this. Read Dan Ariely or Robert Cialdini, who is the guru of influence. The number one way to influence someone is to do a favour for them or give them a gift.
That's why, when you're at a restaurant, your server gives you some mints. It increases the tip you give, on average. That's why all sorts of people send out envelopes with free fridge magnets. They've given you something and you feel you have to give something in return, so you make a donation.