This post of Tim Holman is absolutely worth reading.
Tim presents the elementary basics on the People and Process level for sharing of classified data with trusted third parties. From my point of view these basic principles must be applied for handling of classified data inside a company as well.
In particular for strictly confidential classified information I would strongly recommend to take further actions:
Review of all authorizations and permissions with strict regards to the Need-to-Know and the Separation of Duties principle.
Reorganization of all filing structures
Both measures can be implemented rapidly and will raise the overall level of security because we know in detail who is authorized to access the information and where the information is stored.
In addition technical measures like an integrated Tagging/DLP solution could be applied to support the employees in enforcing the company’s security policy. In my opinion encryption is the last line of defence.