Police departments in the US seem to have a really hard time doing this. They create structures that, rather than encouraging bad apples to turn good, encourage good apples to turn bad. There is shockingly little accountability and even less transparency. There's an overpowering culture of looking the other way when abuses happen, and of never, ever, ever ratting out a fellow officer, no matter how egregious the misconduct. There's an internal set of ethical mores that prioritizes supporting fellow officers over the well-being of the community they police. In an environment like that, even saints will quickly tend to become corrupt.
! And I believe that will continue so long as police remain in their own insular culture apart from the community at large. Breaking down the walls of mistrust and suspicion is not just a one way street either. There are barriers to communication and understanding on both sides and not only do the police need to let down their guard, but the community at large needs to reach out to the individual police officers. Unfortunately latent racism and unrecognized cultural differences get involved at that point and only make solutions more difficult. The trick is finding common ground to bridge those barriers and both sides have to be looking for that common ground.
