I seem to remember years ago watching the Oprah show, talking about self defense. Others above are correct in that showing confidence, in your walk and in eye contact counts for a lot. Also never setting aside your intuition. If your gut tells you something is amiss in a situation, then please pay attention to instinctive fight or flee.
As a young woman, I had a man lurch towards me and try to grab me at a shopping mall. This was during pre-Christmas week, and I instinctively screamed. He ran away with his friends, and I seem to remember all the shoppers turned and stared at me, but no-one chased or confronted the man. I was on a lunch break, and filed a police report. It turned out the young men were grabbing women and trying to kiss them. No charges were pressed, but this would be seen as sexual assault now. It was quite traumatizing, and luckily not serious.
I try to be aware, and not go out of the way places on my own, but don't live my life in fear of attack. I'm afraid that drugs have changed cities now, with so many unpredictable instances of violence.