I think a good first step to policing the police is to have any use of violence by law enforcement be put to trial in court. They would have all of the same constitutional protections as any other defendant and "I was an officer of the law carrying out my duty" would be a reasonable mitigating factor. There would be no need to jail them or require bond or arraignment or any of that, but they would have to show up for the trial and demonstrate why use of force was necessary.