A smaller .pst will allow Outlook to run faster. If you empty your deleted
items folder and then archive olds messages, it creates an archive.pst file.
Remember, you can open any .pst file at any time with file - open ... you
don't need to import files into the current pst file to view the old ones
(and, in fact, that's to be discouraged.) Here's some related stuff from
the help menu.
About Archive and AutoArchive
Your Outlook mailbox grows as items are created in the same way that papers
pile up on your desk. In the paper-based world, you can occasionally shuffle
through your documents and store those that are important but not frequently
used. Documents that are less important, such as newspapers and magazines,
you can discard based on their age.
You can quickly complete the same process in Outlook. You can manually
transfer old items to a storage file by clicking Archive on the File menu,
or you can have old items automatically transferred by using AutoArchive.
Items are considered old when they reach the age you specify. With
AutoArchive, you can either delete or move old items. Outlook can archive
all types of items, but it can only locate files that are stored in a e-mail
folder, such as a Microsoft Excel spreadsheet or Word document attached to
an e-mail message. A file that is not stored in a e-mail folder cannot be
archived.
AutoArchive is a two-step process. First, you turn on AutoArchive. On the
Tools menu, click Options, click the Other tab, and then click AutoArchive.
Second, you set the AutoArchive properties for each folder that you want
archived. At the folder level, you can determine which items are archived,
and how often they are archived. You can automatically archive individual
folders, groups of folders, or all Outlook folders. The process runs
automatically whenever you start Outlook. The AutoArchive properties of each
folder are checked by date, and old items are moved to your archive file.
Items in the Deleted Items folder are deleted.
Several Outlook folders are set up with AutoArchive turned on. These folders
and their default aging periods are Calendar (6 months), Tasks (6 months),
Journal (6 months), Sent Items (2 months), and Deleted Items (2 months).
Inbox, Notes, Contacts, and Drafts do not have AutoArchive activated
automatically.
There is a difference between exporting and archiving. When you archive, the
original items are copied to the archive file, and then removed from the
current folder. When you export, the original items are copied to the export
file, but are not removed from the current folder. In addition, you can only
archive one file type, a personal folder file, but you can export many file
types.
When you archive, your existing folder structure is maintained in your new
archive file. If there is a parent folder above the folder you chose to
archive, the parent folder is created in the archive file, but items within
the parent folder are not archived. In this way, an identical folder
structure exists between the archive file and your mailbox. Folders are left
in place after being archived, even if they are empty.