Employing a system monitoring program where the HR person or compliance officer can replay the behavior of an insider is invaluable. Combine that with data loss prevention (DLP) technology where you can set rules and, based on those rules, block content that you do not want to leave the network.
Perimeter CTO Kevin Prince has kindly offered up several tips for preventing a data breach. Kevin says, "Being aware is always the first step. Taking a layered security approach and looking for the policies, procedures, and solutions that can best mitigate security threats is best."