How to move an offline Outlook Data File (.ost)

Brief summary:

1. Close Outlook, open Mail in Control Panel.
2. Disable Cached Exchange Mode for the account.
3. Click Disable Offline Use on data file.
4. Use the Browse… button to choose the new location fo the .ost file (you can move the old one to save time)
5. Re-enable Cached Exchange Mode.

Read the detailed instructions here.