When I was in the university teaching martial arts about 10 years ago. One of the big focuses and concerns was how to fight more than one person at a time. Realisitically if you are a bigger guy like myself, no one is going to fight you unless they feel they have the advantage. A weapon, friends or both. So in my experience with self defense I have rarely been in a one one one fight situation. There is always at least one friend hanging out in the background.
The first and most important step is a tactic that I take from ninjitsu which is awareness of your surroundings. You have to know how many exits you have, how many exits you can create, what could be utilized to your advantage. Of course when you are in the middle of a fight it is too late to analize these things. So it should be a habit that anywhere you go you are "counting" your environment. In most cases you will find you can see a potential situation developing before it escalates and your already gone.
If you are really in a life or death situation then your biggest concern should be survival. So it is important to train your movement aside from combat. Running, rolling, climbing etc. You could have the most deadly punch in the world but if you can't get from A to B it's worthless. Aside from basic mobility it is also important to learn how to stack the deck in your favor. For example if you put a table between you and your opponent you have given your self a sheild from his attacks but placed yourself in perfect kicking postion.
Also it is important to realize that you have the advantage because you have the element of surprise. Unless you are fighting against a tactical strike team which is organized and coordinated with their attack. In that case....good luck.