Situational awareness is one of the most important self-defense techniques. With good situational awareness, you will be able to sense trouble before it finds you.
The great thing is if you work at it enough, you can learn to heighten your awareness through exercises.
One of the best techniques involves your sense of sight. When you use the technique you mentally break the environment up into the foreground, middle ground, and background. This technique can be used anywhere and should be practiced often.