To find out what is taking up space
Right Click Main Mailbox and Click Properties
Click the Folder Size Button
That will give you an idea what needs to be archived
If you have folders with old data you will want to move those to an archive pst.
I would use a usb hard drive and name it firstname.lastname.11-23-09(todaysdate)
to create the file go to outlook
File -> New -> Outlook Data File
Name it firstname.lastname.11-23-09(todaysdate) for the filename and the description
Put in the info and now it should show up in the list
Now you can move stuff there.
Archiving… Sent Items as Example
Click on Sent Items folder
Click File Menu -> Then Archive
Select the PST file on the USB Hard Drive
Set your date to archive I put 2 years before today
Then Click OK to start it should show archiving in status bar.