Judicious Reciprocity
We are wired to be social. Every time you give something free, you get something back. The receiver feels indebted to you and wants to bring the equation back to par. How can they do that? By buying from you or reciprocating in some way.
In his book Influence, Robert Cialdini talks about how volunteers of ISKCON used the principle of reciprocity to increase donations by offering strangers a flower or a copy of the Bhagwad Gita. When people tried to return the 'gift', they'd refuse to take the gift back and ask for donations instead. Eventually, people became wise to their ways and started avoiding the ISKCON volunteers and ISKCON had to file for bankruptcy in the USA.
Remember to use these weapons of mass persuasion judiciously!