[dba-SQLServer] Who's using my db?

Does anyone have any suggestions on how to track/view a log of users that have accessed information on the Server and specifically at the  Database level?

There are users authorized to view a particular db with confidential information, and I need to verify that no other users are accessing this data.  In this situation it's rather complex because security keeps out the majority of people, but there are certain people with the sa password and admin rights on the server that should not be looking either.

Thanks in advance,
Chris Mackin