This hint will save you a lot of time if you archive your email from last year, quarter, etc. This applies to Apple's Mail.app program only as far as I can tell.
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!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050105164804763