This really cool video by Amy Cuddy (a researcher at Harvard) talks about body language and power.
Amy Cuddy: Power Poses from PopTech on Vimeo.
You can feel powerful by expansive body language. Cuddy's work is placed mostly in a business context, but it is easy to translate this to personal safety and self-defense. Strong body language helps deter assault, and makes you stronger in case you do need to defend yourself.
Body language is a key component of my self-defense classes. We focus on a few aspects, mostly eye contact and smiling, where to place your feet, and posture. These critical elements of non-verbal communication convey an impression of power, and are your first line in physical skills self-defense.