Using Andreas Amann's Mail Scripts, you can archive mail based on date (as well as do a lot of other things). If you select the entire folder structure, there is even an option to preserve the folder hierarchy. Nice. So my 2004 emails are archived in a folder I named "2004," with the entire folder structure preserved within it.
Oh, and unlike most other archiving solutions, it doesn't require FileMaker and it's free. Way to go Andreas!

