Paladin and Jesse are right. It is more than possible to comprehensively train a soldier to the point where they are equiped to act according to their training without shitting themselves in a high stress scenario. A good example to add to the several good examples listed by those guys can be seen from the training of the SAS. One of the drills they do is in a room they call the killing room. One member of their squad will sit on a chair in the middle of a blacked out room, with targets (no coincidence that targets nowadays are pictures of men, either) all around. The rest of the squad then practise live fire drills, bursting into the room, using flash grenades etc, along with shooting live ammunition at the targets around the member of their squad. This kind of thing trains individuals to function effectively in extremely high stress environments. It also trains those who are sitting on the chair to remain calm when bullets are flying all around them and they are disorientated. Do not underestimate how possible it is to train yourself, within a controlled environment, to be a genuine force to be reckoned with, someone who is capable of prevailing when things go south rapidly. It is possible, and it happens every day.
That said, for civilians, nothing so drastic is required. Spend some time living somewhere rough and you will quickly find that you instinctively recognise trouble, potential threats etc. Learn the body language, avoid areas you know are dangerous and realise that if running is an option, running is your best option. At the point you actually have to fight, you have fucked up pretty badly, because unless you're jumped, you've failed to notice clear warning signs.
If, for any reason, you do actually have to fight, my personal belief is that your mentality and resolve, rather than any fancy tricks, will be the decidinng factor. If you find yourself unable to run, you need to realise you're in a fight before the guy in front of you realises he is in a fight. Hit first, hit hard, then run. Personally, I believe sport fighting where you learn to strike effectively, coupled with some conscious and practised alterationis to disguise what you are doing in an altercation, is as effective as most things.
Violence is predictable, and not often random. You are either having to fight because you failed to notice trouble coming your way, or you have actively solicited trouble by letting your ego get in the way of your good sense. I don't personally believe there is a great deal of merit in learning to disarm people with weapons, for a number of reasons. If someone gets into lethal range of you with a weapon, you have missed a glaring opportunity for escape somewhere along the line. If you cannot recognise someone who is headed your way with bad intentions, I don't think that a few disarming techniques will do you any good. The good news is that if this applies to you, you probably haven't upset the person who is armed, in which case, your best bet is compliance. Noone wanting a wallet, or your car, wants a murder charge if they can get away with a theft charge.
If the person coming at you with a weapon is known to you, because you have got yourself mixed up in some funny business, then the chances are he has arranged a situation where he knows you're fucked. When someone like that holds all the aces, you really dont stand a chance, because he is intent on harm, usually has back up, and will have planned the situation to catch you unawares. No technique in the world will save you in such a situation. If you are at risk, any risk, of comeback from a falling out, it is my personal belief that the only effective techniques are to strike pre-emptively, and in such a way as to ensure that the desire for revenge is extinguished in your adversary, or to pack up your life and move far away. Anything in between and you stand a good chance of being a statistic. Better to be judged by 12 than carried by 6. This is something that everyone who anticipates dealing with violence must force themselves to come to terms with. It is extremely unpalettable, but it is the reality of illegal, street violence. Hence the emphasis posters such as Paladin and Jesse put on avoidance as the most effective self defence technique.
"My name is Smokin' Joe Frazier, sharp as a razor! Ha! Yeah, "floats like a butterfly, stings like a bee". Ha! I'm the man who done the job. He knows. Look and see. Call me, bye-bye"
When You Go Home, Tell Them Of Us And Say,
For Their Tomorrow, We Gave Our Today