First, stay calm and think as the situation develops. As soon as the adrenaline kicks in, everything will seem to happen in slow motion. If you are calm, if you do not panic, your mind will process thoughts so rapidly that it will seem like you have hours to make a decision about how to react.
Second, the human skull is an awesomely powerful weapon. Bashing your forehead into the goon's nose once is tremendously more effective than slamming your fist into his nose twice.

Similarly, ladies, if you are grabbed, bear-hug style from behind, don't waste your time trying to step on his toes, or elbowing his ribs, or kicking your heel up into his groin. It is highly unlikely any of hose moves will do anything besides anger your attacker.

Instead, start trying to bash his face with the back of your head. All you have to do is connect once or twice with your attacker's face or collarbone and you have delivered some serious damage.

Always, always, always have something easily and quickly accessible to use as a weapon. If someone surprises you, there should be something instantly accessible to aid in your defense.

If you remember this one absurdly simple rule about weapons fighting, you will see the potential weapon hidden in virtually everything around you AND be able to use it more effectively.

Anything you find that is hard and fast should be aimed at smashing against something made of bone, and anything with a point to it should be aimed at stabbing into soft tissue. For example, if you found a blunt stick or a can of vegetables you would target bone: Aim this kind of weapon at the face, the skull, hip, shin, elbow, or kneecap. However, anything hard or blunt would be less effective to use against, say, an attacker's abdomen.

Conversely, something with a point -- a knife or pen for example --  is much more effective when targeting something soft, like the throat, the eyes, crotch, armpit, or belly.  If you do strike at something hard, like the kneecap, chances are the point will bounce off without doing any real damage.
Hard goes to bone, 
Point goes to soft tissue --  
It's as simple as that.  Remember this rule, and you will never be without an effective weapon again.

(a bit of theory)
There is one tip about self-defense that is so important that entire martial arts systems are based upon it.

Don't get hit! 

Moving along a triangle goes a long way toward achieving the goal of not getting hit. One of the most dangerous mistakes the average person makes during a fight is to move in straight lines.  They will move in a straight line, either forward and backward, or side to side.

This is also the mistake that will cause the Tai Mai Shu black belt to get his or her butt whooped in very short order out on the street.

Imagine a vertical dividing line along your body, dividing your body into left and right halves. The aggressor is probably going to attack some point along or around that line: your face, your throat, your heart, your groin.

Moving in a straight line backward or forward will change the distance you are from your attacker, but it does not move your centerline out of the attack path. Moving laterally (left or right) will change the location of your centerline, but it does not change the distance between you and your attacker.

Your attacker has mentally committed to striking at a particular target. His brain has sent the signal to his fist that your face, your throat, your heart, or your groin (the target he intends to hit) is located at a particular distance out there in a particular direction. When you change the target's coordinates, it spoils the effectiveness of the attack.

Your goal is to move that line of your body out of the path of the attack AND change the distance of the target from your attacker. Your attacker may be able to recover from a change in target location or a change in target distance alone, but changing both factors is your best bet. Then, even if it does connect, the strength of the attack will be greatly diminished.
Moving along an imaginary triangle changes BOTH.

Imagine standing with both feet on the pointed end of a triangle and facing the bad guy. The other two points of the triangle can either be in front of you or behind you.  
Each of the other triangle points are only about one medium-large step away from where you are now.  One point is found one step forward and to the left.  Then there's another point one step forward and to the right.  Behind you one point of the triangle is one step backward and to the left.  The other point is one step backward and to the right.
All you have to do is step one foot onto either of the two available triangle points in front of you or behind you. What have you done to the distance to and the location of the attacker's original target?
You have changed BOTH your direction, and your distance.
Simply bring your other foot up, and you are now at the starting point of another triangle.  Use this concept every time you move and you will continue to confuse your attacker.

During a fight, as during a game of chess, the experienced player is already planning the second or third move before the first one is ever completed. In fact, many of the experienced fighters' moves are used solely to get the opponent to react in a predetermined manner.
Fight you own instinct and do not back up. 
Your instinct is wrong!
For example, imagine I am throwing a flurry of jabs at you.  In my mind, I "know" exactly what you are going to do: backpedal to escape my vicious attack.
In fact, I am counting upon you backpedaling into that corner behind you, then I'll pound you into a liquid, right? How surprised am I going to be when you step forward, along your trusty triangle, and not backward? 
I would be very surprised because you are not "supposed" to step into a savage attack; You are "supposed" to step away from it.
Look at this scenario.  You've just stepped forward along the triangle. While your attacker is busy trying to adjust his thinking to handle this unexpected event you are now inside his defenses.  You now have access to his unprotected ribs, armpit, neck, head, abdomen, flank, and knee -- Suddenly YOU have a virtual smorgasbord of targets. 
That's when you slip back to using tip #2: 
Smash anything HARD against something made of bone, 
Strike anything you have with a point at something soft.
All of these tips are simple common sense.  If you are smart you'll never have to use them because SMART people never put themselves in situations which may become violent.  
As I see it, the goal of your self-defense training is to have the ability to utterly destroy another person, but the foresight to avoid situations where you might have to demonstrate that ability.
I want you to carry these tips in your hip pocket, 
just in case -- just in case.