Thanks to Dave Street for this great quote from an article in the Guardian:
"Rule 1 You've got to have a moral ground. You've got to know the difference between what's right and what's wrong. A policeman being kicked around: that's wrong.
Rule 2 Get the attacker's attention away from the person that is being attacked. Shout at the attackers. Before you throw a punch, try to distract or intimidate them.
Rule 3 If there are other people around, get them involved. Don't try to be a superhero - work as a team.
Rule 4 If you have to use physical force, think about what you are doing. In hindsight, I should have rugby- tackled the attacker, rather than kicking at him. Then we could have all pinned him to the ground.
Rule 5 There are no rules to being a have-a-go hero. You've got to go with what your natural instinct tells you. If your gut feeling is telling you to get stuck in, get stuck in. If it tells you to run, just run. Hindsight is a wonderful thing, but it won't help you now."