Whenever I tried to look into such incidence I see that prosecution gets stopped (or didn't even start) at a local level and the whole process stinks of conflict of interest. Asking local police and prosecutors to investigate misconduct just seems like a bad idea.
My solution would be to automatically throw every violent police incident to a special-purpose federal body which is not otherwise involved in enforcing laws. This would be filled with prosecutors focused on civil rights and those people would hopefully be highly motivated to prove their case in court.
The only problem I see is that this it ignores states rights but those are not particularly popular anyway.