When we think of self defense we normally think of throwing punches or a well placed kick to the groin. But fighting in the real world isn't like fighting in the movies (no kidding, right?). Most of the time you'll find that fights immediately get too up close and personal for things like kicks and punches.
When that happens and your opponent gets in too close, it's time to start throwing elbows. Just bend you arm in and drive your elbow toward your opponent. Elbow strikes tend to be overlooked by amateurs but I've got 2 reasons why you need to be paying attention to them if you find yourself in a bad situation.
Now I'm not encouraging people to go out and use elbow strikes against anyone they feel like. Fighting really should be the last option. But if you find yourself in a situation that you just can't seem to get out of peacefully, then don't hesitate.
Start practicing your elbow strikes on a heavy bag next time you're in the gym. They're always more powerful and effective when they're used by a trained professional, but even a sloppy amateur elbow in the right place can do wonders for ending a fight fast. You'll drastically improve your power and strength no matter how big your opponent may be.