A client wanted to clean up his emails so I suggested the following:

1. In his Main Identity, Empty Deleted Items, then Quit.
2. In the folder Microsoft User Data:Office 2011 Identities, Duplicate the Main Identity folder, then rename the copy Old Email or whatever you wish (in case you need to go back and find an old email).
3. Open Outlook (should default to Main Identity), go to Advanced Find, and Find emails in All Mailboxes where Date Sent is greater than X days old, where X is the # of days of email you wish to keep in the Main Identity (e.g. 365 for the past year). [In my client's example, he had over 75,000).
4. *VERY IMPORTANT* Go to Spotlight Preferences:Privacy and Add the Microsoft User Data folder to the list. This will prevent Spotlight from slowing down the email deletion process by reindexing them.
5. Back in Outlook, Select All, then choose Edit:Delete. All will be moved to the Deleted Items. Select Deleted Items folder, then control-click to get the popup menu, and Empty Folder.
6. Now you'll need to repeat the process on the other database, but first you'll need to delete the Microsoft User Data folder from the Spotlight Privacy wiindow so it will Find properly (otherwise Outlook Find doesn't work!).
7. Quit Outlook, open Microsoft Database Utility (option-Outlook), and choose the other (Old Email) folder and choose Make Default.
8. Open Outlook, which should open the Old Email, and immediately go to Outlook:Work Offline. Then go to Tools:Run Schedule:Edit Schedules, and uncheck all of the schedules (e.g. to check mail, etc.). This will prevent this identity from automatically downloading recent emails, which you don't want.
   
9. Then go to Advanced Find, and Find emails in All Mailboxes where Date Sent is LESS THAN X days. This finds all of the more recent emails.
10. Again go to Spotlight and add Microsoft User Data to the Privacy window.
11. Again in Outlook, Select All and Delete/Empty Deleted Items. One that's done, you can choose Outlook:Work Online.
    
12. Quit Outlook, open up the Microsoft Database Utility, then select Main Identity and Rebuild, then Old Email and Rebuild. Then select Main Identity and choose Make Default, then quit.
13. Again delete the Microsoft User Data folder from the Spotlight:Privacy window to allow Spotlight to index your emails.
14. In Office 2011 Identities, Trash both of the [Backup] identity folders, leaving the two that you just finished optimizing.
15. Open Outlook to Main Identity, and carry on.