It doesn't have to be formal. Everyone does this today, and keeps a kind of list of debts in their head. "Bob asked me to help him move this weekend, and I rea

It doesn't have to be formal. Everyone does this today, and keeps a kind of list of debts in their head. "Bob asked me to help him move this weekend, and I really don't want to, but he helped me move last year." If you don't repay your social debts, you're going to find yourself getting a lot less favors done for you, and having a lot less friends in general.