Depending on the format of the PST (2000/2003, 2007, 2010) and the size, this process can take a very long time. I worked at a place where the hit the old 4GB cap regularly and would have five or six of these (because they never ever deleted anything). I would routinely have to start them at night as the user was leaving, and it would usually be done by morning.
Follow these amazing and simple steps: 1. Delete any items that you do not want to keep and empty the Deleted Items folder. 2. Click File > Account Settings >Account Settings. 3. On the Data Files tab, click the data file that you want to compact, and then click Settings.