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.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.