Take people you spend time with to first aid, heimlich, and CPR classes. You will need their help if you are the one choking or unable to breathe.

Build up enough stamina and physical fitness to run at both a sprint and for several minutes straight. Running away from a fight can be a very good strategy for not getting harmed.

Read the "Influence: Science and Practice" chapter that discusses Social Proof. I think it was chapter 4. The suggestions involved help avoid the bystander effect where a person in need is left alone and unassisted by a group of onlookers. The chapter deals with a few examples of effectively communicating and prompting someone to help you in an emergency situation. This is especially necessary in an urban environment.