I would also recommend *not* requiring you to *unsubscribe* (as sometimes such things don't even work or nag you further).
Why not simply automatically unsubscribe someone after they fail to donate 2 or 3 times (with the last contact reminding them how to re-subscribe)?
Because someone might want to skip a few months for whatever reason, without stopping the reminders. It would make no sense to try and 'predict' what someone wants, I'd rather let them choose for themselves
