How many times did you helped users to cleanup their mailbox? The usual way of cleaning up the mailbox is to archive the old mail, cleaning up the Sent Items, moving mails to PST folders, etc.,
Do you know there is Mailbox Cleanup utility comes with Outlook 2007? If you don’t know check it out. I would strongly suggest educating the users about this tool in Outlook and let them clean up their mailbox themselves. If they do it, it’s really Cool! Isn’t? 
To invoke the Outlook Mailbox Cleanup tool, Click Tools –> Mailbox Cleanup.
It looks like this:
Anand, the Architect 