I keep saying this, hoping the idea will catch on...
Make it a Federal Law that all Law Enforcement Officers must have on their persons active video cameras recording their actions at all times while on duty. Make it part of the required uniform, as essential to an officer as a badge.
Make it a major crime to tamper with or disable such cameras or the video they record. Criminalize the act of failing to maintain such cameras, or otherwise allowing them to become inoperative through negligence. Require the cameras to not only store their data locally, but to also remotely upload it to more secure storage devices, for the sake of redundancy. Hold everyone involved in the process of data retention responsible for "lost" or "missing" footage and data.
Cameras are cheaper, smaller, and higher quality than ever. If the police can afford to train and equip SWAT Teams in podunk backwater towns across the nation, they can damn well afford to put cameras on every gun and integrate them into every uniform.
Too many court cases come down to unsubstantiated claims of witnesses and participants lacking material evidence - so why the hell aren't we requiring the police to produce material evidence? We have the technology to do so with little trouble!