Have every single cop wear a device that records what they do (headcam type thing, something like that) which also records sound, and have that uploaded to a remote server. If this recording is EVER turned off or blocked, anything anyone says against the police will be take with more weight than what the police say themselves.
This in itself invites problems, of someone trying to cover up said camera so that the cop's word doesn't count. Thus, the voice recorder should be separate, perhaps have one 'obvious' camera and one non-obvious one on the cop, and in general that means it's just one of the dangers of the job if the cop decides to stand still when someone's about to throw a bucket of paint on him while playing 'Thunderstruck' at 140 decibels to drown out the recorder.
But in general... have something on the cops to record what THEY do, and make it illegal for THAT to ever be tampered with. And make it publicly available instantly online, since y'know... we pay their bills and they're doing a public service so therefore everything they do should be visible by us.