We use Outlook PST files to archive our emails. From experience, expanding their mailbox quota will only mean the users will hold more email in their mailbox and decide it is time to archive when they are close the the new limit.
We use Symantec’s EnterpriseVault to archive e-mail to an EMC Centera. We allow the users to keep 6 months of data in their Exchange mailbox & thenit is automatically removed and archived. There is a plugin we installed that lets them read the archived e-mail and restore it if necessary.
We use native Notes/Domino archiving, both server and local archives. It’s easy to set up and it works.
People who choose local archiving have the added benefit of being able to get to/read their archives even when their off line.
We use Outlook PST files to archive our emails plus we leave a copy of the e-mail at hosting company server. Very simple way to archive.
We use native Lotus Domino archiving capabilities.
In addition, in order to extract emails and store them with client/case/project related files, I can recommend SWING PDF Converter (take a look at the screencast: Lotus Notes email archiving).
Sunbelt Software Exchange Archiver.