To truely and effectively prevent leaks, be a hermit. Simply operate on your own, trust no one... and thus employ no one to work for you and never be networked to anyone.
Same principle of avoiding computer viruses is not to have ports to accept input devices, not to be networked and always be alone.
To elaborate, don't employ government contractors or workers. Don't have senate or statesman. You are yourself alone. That is the best to prevent leaks.