The Hotmail Junkmail Reporting Program

Yep, that's a mouthfull. Hotmail has a great program for removing people who think you're a spammer from your lists. The mechanics are really simple, whenever some junks your e-mail, you get an e-mail notification of this event. You should take this notification and put the user on your suppression lists. That's it.
To make this work, you should put some sort of code in your mail (in the headers, for example) that will help you identify the person who's junking your mail. If you're using multiple list, make sure you can identify the list as well. Reserve a dedicated e-mail address to receive these reports and make sure they get processed.
This kind of program is called a feedback loop.

Other providers

Several other providers provide feedback loops as well. For an up-to-date list, check out It's worth it!