What's your strategy for archiving emails in Mail.app? Do you archive your Sent & Trashed emails too?
I use Mail app's "smart mailboxes" instead of copying and moving the actual messages in Mail. 
For example, to archive July 2011:


create a new smart mailbox, 
name it "Mail Archive July 2011" or whatever, 
set the find criteria to "Date Received is in the date range 1 Jul 2011 to 31 Jul 2011",
you will probably want to check the "Include messages from Sent"
Hit okay and in a moment you have a folder contain all of July's email
Right-click the smart mailbox and choose the "Archive" option and save.


After this you have the option of keeping the smart folder or deleting it. Spotlight (in OS 10.6.8) searches the .mbox archives Mail produces, so your archive remains searchable.