Here's the trick. The gift does not have to be big and expensive... In fact it shouldn't be. Humans are compelled to return favors, even if they are very small. Perhaps you remember the guy who washed your car windshield and asked for a handout? Or how about the junk mail that arrived with free address labels along with a request to donate to a specific charity? Or the free lolly pop at the bank? Or the free coffee mug that you received for signing up for another credit card? These small gifts aren't really free. They are carefully chosen tools of manipulation.
Professional marketers are well aware of the power of reciprocity and the act doesn't have to be secret or disguised. Studies have shown that doctors are more likely to prescribe specific medications when provided with free meals or other trinkets from the associated pharmaceutical company. Reciprocity is a very powerful tool.