The Roman poet Juvenal once asked "Quis custodiet ipsos custodes?", Latin for "Who will guard the guards themselves?" Referring to the ever-recursive problem of corrupt police and other officials. The police are expected to protect you against crime, but if the police themselves are corrupt, then society is just sort of out of luck on that aspect. Worrisome, as there are two kinds of people who want to be police officers. One type wants to be helpful and bring peace to the community, and should be encouraged, but the other loves power, and to boss people around, and should be discouraged. (The second kind is also deeply corruptible, further harming everything around them.)
In most modern police forms, this is answered with a police-of-police department, called Internal Affairs. Their job is to investigate complaints against police officers, and arrest those who engage in illegal acts. The Internal Affairs department is unpopular with the police for the same reason that police are unpopular with the general public: they're the ones telling you that no, you can't, and the rules apply every time with no chance of exceptions. No you can't plant evidence on a guy you don't like. No, you can't beat up people who pester you. No, you can't except bribes. Not even this one time. (Just as the regular police tell you that no, you cannot have a loud party at 3am. Not even just this once.)
The other day I was thinking about this, and jury duty, which is expected of the American population, and this gave me an idea. What if, in communities with low trust for the police, we call in random people for police-oversight duty? When called, you become an internal affairs officer for the day, investigating and rooting out corruption. Random people will gain insight into police activity, why they do as they do, and police officers will have every incentive to do their jobs properly and professionally. The watchers will be watched themselves...by everyone.
Probably excessive, as I hear that Internal Affairs does a good enough job as it is.