by Steven J. Owens (unless otherwise attributed)
"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing." -- Helen Keller
Keeping this quote in mind, self-defense is mostly about awareness and anticipation. There are two good reasons for this.
First, you can learn all you want about physical or gun combat and still have your ass handed to you if you don't learn to pay attention to what's going on around you, anticipate and avoid danger. Even a green beret or navy seal who spends a lifetime training for combat will get taken if they're asleep on their feet (they won't be, of course, not if they're the kind of people who end up becoming green berets and navy seals :-).
Second, criminals do not generally operate according to a master plan, unless it's the master plan of "impulsively take advantage of easy opportunities." Let's face it, generally an honest life is a lot less risky and usually more profitable. But you have to have the forethought to notice that and the discipline to practice it. Criminals don't go down a row of brand new parked cars, pick one, then work at it until they get the lock open. They go down the row of cars until they find one with the doors left unlocked. Car theft statistics show a sharp rise in thefts during the summer - because it's easiest just to bash the window in and then drive with the shattered window rolled down.
You don't have to be the toughest dude around; criminals will choose the easiest target, so don't be the easiest target. Or, as the old joke goes:
"I don't have to out-run the bear, I just have to out-run you."
You might want to take some self-defense courses. Certainly, I feel that most folks ought to learn some basic personal combat skills, armed and unarmed, just like basic gun safety, basic first aid, basic driving skills (in the US at least), etc. You may not need them on a daily basis, you may never need them, but the cost/benefit ratio for at least basic training is pretty persuasive. Also, I suspect that some physical training will alter your presentation and body language a little, which will make you look less like a victim. But the real payoff will be in developing awareness.
The best way I've found to describe this is:
Train yourself in habits of awareness.
Remember the Helen Keller quote above; none of this means you should go through life feeling fear and being paranoid. That's why they're habits instead of an activity to which you devote your waking hours.
First, you need to acquire the knowledge to understand where the risks are. But then you need to practice the techniques until they become habits of thinking - you need to look around you, you need to habitually analyze the self-defense risks around you. At first this requires you to actively analyze things, but after a while you can develop the habit of noticing, understanding, and taking steps to minimize self-defense risks.
Here are some simple examples, but I'm not trying to teach a self-defense course here; first of all, for god's sake, if you want to learn self-defense, find a real self-defense course and instructor. Second, I'm just trying to illustrate some principles. You should focus not on the action, but on the principles that drive the action.