One of my all time favorites MailSteward has updated, so that now you can schedule it to archive your email automatically.
“Archives can be scheduled either by days of the week, or by months and days of the month, and they can be scheduled for any specific time of day. After setting the scheduling options, clicking on the Schedule button will put an entry into your crontab file, which will tell the operating system to run MailSteward at the specified time and dates. MailSteward will then automatically archive email beginning on the date of the last email in the database file. As always, any duplicates will be skipped. The scheduling options can be changed at any time. Changing the options will replace the MailSteward entry in the crontab file.”
I have reviewed the application several times before so I wont do that here, suffice to say that I think you could do a lot worse than take a look at their demo.