I mostly give locally, and have had good luck limiting the reminders and mailings by simply asking that they stop. My local public radio/tv station is a good example of that. Having been on a not for profit board of a local chapter of one of the biggest national charities, I can tell you that the pressure to raise money is intense and never-ending. I often felt that we were employing less than ideal methods because of that pressure. If an organization you otherwise support has a practice that you find obnoxious or troublesome, I'd encourage you to share your thoughts with them. If it helps them figure out best practices, then you will be doing them a favor in the long run.