Bystander gets it right with these nine points. They're not new, but they stand the test of time. Although I sometimes have trouble with this one, which means it is a good thing I am not running the Police force.
To use physical force only when the exercise of persuasion, advice and warning is found to be insufficient to obtain public co-operation to an extent necessary to secure observance of law or to restore order, and to use only the minimum degree of physical force which is necessary on any particular occasion for achieving a police objective.
I'm more of a maximum force kind of guy. I've never understood for instance why we stopped whipping thieves.