Once leaving government service whether military or otherwise security clearances should be removed. They can be re-instated if it is deemed by congress to benefit the safety and security of the United States.
This will limit spreading classified information to others and cut down on the power of lobbyists. This will also provide better transparency as to who is in government service. I.E. NGOs, thinktanks, and other organizations who actually do work for the government but are not officially connected. This would also include people that write the laws that are submitted to congress.