Use your Voice. First and foremost, you need to make it clear to potential aggressors that you have boundaries and if they are crossed, you will defend yourself. "No", "stop", "don't touch me", and "don't come any closer" said in a loud and commanding tone (think your best "Bad Dog" voice) can dissuade the less committed. Additionally, if you do get in an altercation, it is IMPERATIVE that you get as loud as possible. I tell students all the time, "If you fight silently, you fight alone". Use your voice, and help could be just around the corner.