How many of those e-mail attachments are really worth keeping? (In the sent
folder and various Inbox subfolders.) If you have already saved the
attachment, delete it from the e-mail.
If you really need to keep all the mail, why don't you archive? You can
open the archive folder and still access it in the same way. You can then
either archive or drag and drop any mail that you don't need in the mail
folder into the archive folder and keep the Outlook.pst one small.
I set up a new archive once a month - it keeps everything tidy. It also
makes it much easier to backup.
Remove all attachments and save them on your hard drive if you need them.