Security needs to be a constant part of an employee's training as much as cultural sensitivity training or other human resources mandates to minimize accidental employee involvement in data loss.
Segregation of duties policies must also be established to limit the roles and responsibilities of users and the activities they perform only to those they have permissions to do.
User activity must be logged and audited on a regular basis to ensure that potentially malicious activity is detected and acted upon on a timely basis. Centralized logging and security event management applications allow security staff to collect data from a large number of sources, correlate the information, and detect events of interest related to insider activity. File integrity monitoring detects changes to permissions or the addition of new files or removal of existing ones permitting security personnel to track potentially malicious system level activities.
Data leak protection software is designed to monitor network activity for the transit of data through media forbidden by company policy. When it detects sensitive data being e-mailed to an external account, or copied to a USB stick, it alerts the security administrators of the activity.
Many security frameworks like the Center for Internet Security or ISO 27001 are not only designed to deal with external threats from hackers but also take into account internal user activity.