Keep your opponent at arm's length if possible.
Don't use kicks unless you're well-trained in a specific discipline that uses kicks or you have an open target that for whatever reason you can't hit with anything but a kick.
Always keep your core tight. (Practice walking around with your core tight all the time so you don't have to think about it should you enter a fight.)
Keep your hands in front of your face and don't keep them too close or too far away.
Aim for soft spots like eyes, nose, temples, fingers, joints, and genitalia.
If your opponent is armed, bigger/stronger than you, or has friends, and you can't avoid the fight entirely for whatever reason, try to find an equalizer. Also, be armed, bigger/stronger than your opponent, or have friends if you can.