When making a trade in Settlers of Catan, there is no official rule encouraging or prohibiting making a promise (to make a certain type of trade or perform some kind of action in the future) during a current trade. Assuming the group you play with decides to allow such promises, what mechanisms can be used to ensure "future trades" occurs smoothly?
I provide one such answer below that we've used in many games, but I'm totally open to suggestions for improvements on my answer, and/or alternative play-tested systems developed by other Catan enthusiasts.
Note: A similar question was asked that could be considered a duplicate:
See my comment below which advocates keeping this new question open and possibly closing the old question.