Now that the brain is covered lets get down to the moves. All too often people ask me "what do you do if someone does 'this" referring to a punch, kick, or knife attack etc... Same thing happens in dojos, they teach a different technique for a punch, than a knife attack. Now, in the split second (at best) of an attack, you need to notice whether someone has anything in their hand or not. It is a waste of information. "A small handful of well represented techniques is more use full than a large tangled mess of data," Bruce Lee. We should not limit ourselves to one system, but we also should not limit ourselves to getting stabbed because at last second we saw a knife and changed what we where going to do.
You should chose techniques based of the angle of the attack. Many techniques for a punch, can be used for a knife, baton, or in some cases, a gun. Yes you should learn disarms individually, but you should also practise you other maneuvers as if there is a weapon in the opponents hand. I call these, adaptable techniques.
When you pick these out, you should also think of whether you can do them in a less desirable place than a mat or field of grass. Can you do this in a bathroom stall, buss isle, elevator, alley way. Dopes it require you to hit the ground? then maybe you won't want to do this in the street or where there is broken glass that can harm you more than the attacker (if it involves the bad guy hitting the ground that is OK) I can't tell you how many people get surprised to find out hitting pavement with that Jujitsu move they learned hurts them more than the other guy.
There are a number of attacks that do require a non-adaptable technique. This like firearms, submission, choke holds, and many people forget to practise what to do if you are already knocked down or injured.
From here I will refer to a few arts and what they may have to offer.
Aikido: I love this art. everything and anything in it can be adapted for almost any attack. It is difficult to learn however, but most of the wrist locks can be useful even if they aren't performed properly.
Tea Kwon Do: I am not as much of a fan because most of the moves require plenty of distance therefore making most the techniques unsuitable for enclosed spaces, but it does have plenty to offer and is very easy to pickup and keep the reflexes.
Kung Fu (Shaolin specifically): There may be better schools for this but they are few and far between, the only national source for this arts I would trust is United Studios Of Self Defence. I had a blast and learned many great things there, the arts ranges from close quarters, to distance, and ground combat. Very Very good one to learn from and if you like the Dojo environment this is the place.
Karate: A little stiff and structured for me, but it worked against the samurai.
Mui Tai: Simple, powerful, free form but requires some amount of physical conditioning and doesn't really allow for use of weapons "try blocking a knife with your shin."
The most important thing about practising defencive maneuvers, is to train the mind not to go into "panic" mode, and go into "defend, or fight" mode. It takes practise, and creating realistic scenarios without compromising safety. You should always try to find ways to safely bridge the gap between the safety of the Dojo and the danger of the real world.