Wondering how many of you currently have or have worked with campaign frequency preference options for your subscribers, allowing them to choose the frequency that they receive your messages?

This is something that I plan to impliment in the near future, so I'd just like to get a gauge on how it's worked for you and which issues I should be on the lookout for.

One area that I'm concerned with is figuring out a way to easily know who has and hasn't been sent messages during a given timeframe so I know who to mail and who to leave out based on their preference.
We don't have enough messages to allow someone to only sign up for a certain kind of mailing, so we would just need to limit based on frequency.

Interested in anyone's thoughts on the topics.