Sam Harris in his blog:
Principle #1: Avoid dangerous people and dangerous places.
The primary goal of self-defense is to avoid becoming the victim of violence. The best way to do this is to not be where violence is likely to occur. Of course, that’s not always possible—but without question, it is your first and best line of defense. If you visit dangerous neighborhoods at night, or hike alone and unarmed on trails near a big city, or frequent places where drunken young men gather, you are running some obvious risks.
I once knew an experienced martial artist who decided to walk across Central Park late at night. He was aware of the danger, but he thought “I have a black belt in karate. Why shouldn’t I be able to walk wherever I want?” As it happened, this rhetorical question was answered almost immediately: My friend hadn’t ventured more than a hundred yards into the darkness of the park before he was confronted by three men, one of whom plunged a hypodermic needle into his thigh without a word. Our hero bolted and escaped, otherwise unharmed, but he spent the next three months wondering whether he had been infected with HIV, hepatitis, or some other blood-borne disease. (He was fine.) The lesson: Whatever your training, you needn’t be foolish.
Similarly, all men should learn to recognize and shun status-seeking displays of aggression. This is one problem that women generally don’t have to worry about. It is, for instance, very rare for a woman to find herself party to an exchange like this:
“What are you looking at, asshole?”
“Who are you calling an asshole?”
“You, bitch. What are you going to do about it?”
Nevertheless, young men are easily lured into social dominance games from which neither party can find a face-saving exit. The violence that erupts at such moments is as unnecessary as it is predictable. If you want to preserve your health and stay out of prison, you must learn to avoid or defuse conflict of this kind.
More here.