Last night our Singles Group had a self defense demonstration by a retired State Trooper. It was very informative.
While I never felt particularly vulnerable, I never realized I had so many means available for my own protection and defense. He stressed that the main thing to keep in mind when out is to be aware of your surroundings and to have a plan in case of an attack.
I never knew that a head butt could be so effective in protecting yourself. Of course, yelling is the first thing to do, then run, if possible. There were so many other ways of protecting yourself, elbows, knees, feet, etc. You can even use common items such as keys or a pen like the woman who was attacked in a church.
I guess though the main way to keep safe is to avoid certain situations and places, but if that's not possible, have a plan and yell.