How would you enforce a pay-it-forward scheme?

A pay-it-forward scheme works like this. Person A does a good deed for Person B and just asks that, in return, Person B does a good deed for someone. Person B does a good deed for Person C and just asks that, in return, Person C does a good deed for someone. The cycle continues like this, with the goal being to create a flowing river of good deeds.

Pay-it-forward schemes have been a trope for a very, very long time, there was even a bad movie made about it (oh Hollywood, what would we do without you). However, as even the movie acknowledges, they are notorious for eventually fizzling out. If you had the authority or whatever that would allow you to and were to ignite a chain for as long as possible, how would you do it?