In his book "The Zahir",
Paulo Coelho introduces the concept of a "favour bank": a log of favours done for another in the hope (but not expectation!) they will repay them at some point in future. He writes that the first favour should be granted without the expectation of receiving anything in return. But if you can't call on that favour (and let's be reasonable - a night with the debtor's wife is probably not a fair exchange!), then you know not to go out of your way to help that person again.
On the flip side, when a favour is done for you, always keep note of that person's balance in your account.
Keeping a schedule of accounts is one of the qualities that most successful people - in business, politics, the arts and even socially, possess. It's such a simple and rewarding concept, yet one that so many of us ignore.