Should you find yourself attacked, stay calm and think as the situation develops. When adrenaline kicks in, everything will seem to happen in slow motion. Since you are calm, and you do not panic, your mind will process thoughts quickly and it will seem like you have all day to decide on your reaction.
Another thing to remember, your skull is incredibly hard. Use it as a weapon against an attackers' face and/or collarbone.
Most "fights" are won before they start, and aggressors will back down if you maintain eye contact and are not intimidated by them. You won't look like a victim or easy target.
You might have only one possibility to defend yourself, do not lose it.