I feel that self defense is the main reason that the majority of people practice martial arts today. Be aware of all situations that would warrant using it, and avoid those situations at any cost.
Awareness is the key factor in all self defense applications.
Awareness allows you the time to react to virtually any situation. By becoming aware of a possible attack, you have the time to coordinate and prepare your defenses. One very good principle to awareness training is allowing critical distance. Critical distance is your space in a radius of six feet or less around you. Another very good principle is the reflection principle.