1. Go to "File" in the top left of the Outlook window.
2. On the left side, select "Open".
3. In this menu select "Import".
4. In this window, there are a variety of options, towards the top select "Export to a file". Click "Next >".
5. The window will change your options to choose from. Select "Outlook data File (.pst)".
6. The next window allows you to choose what what to include in your .pst file (inbox, sent items, contacts, calendar, etc.)
7. Choose a location to store the .pst file by clicking "Browse...", choose somewhere that is safe and secure, and easy for you to find later on.
8. You can choose to password protect the .pst file, this is optional. If you do choose to password protect, I suggest writing the password down just in case you forget it at a later date.
9. To open the .pst file in Outlook, follow steps 1 & 2, and select "Open Outlook Data File".
For Outlook 2007 insturctions refer to Outlook Quota's and Saving Space