Some things I've found that also help:
When using XP especially, keep the local user temp files cleaned up from:
C:\Documents and Settings\UserID\Local Settings\Temp
Also check:
C:\Documents and Settings\UserID\Local Settings\Temporary Internet Files
\Outlook and \Word folders.
(Make sure applications are closed first.)
Delete old mail profiles.
Keep your hard drive defragged and the same for your PST/OST files.