Most self-defense situations do not suddenly appear, they develop from some minor confrontation that has escalated. If you can nip the confrontation at its bud, it will never blossom into a full-blown, self-defense situation. Most self-defense situations involve the three C's: confrontation, conflict, and combat. You are confronted by an aggressor and there is some sort of conflict or disagreement with the person that leads to conflict.
First, do not put yourself into a situation where self-dense may be required. As the actor James Coburn stated "avoid arse holes and big egos, avoid places where arse holes and big egos hang out."
As the godfather of soul, James Brown, said "I don't know karate, but I know ka-razor!" Knife fighters usually do not show the blade. The first time you see it will be when it is stuck in your body. If an attacker shows you the knife, he or she is usually just posturing. Always check the hands of your antagonist. If you cannot see the palms, or a hand is concealed, assume they have a weapon. If you are facing a knife, the best-case scenario is that you do not die because you will get cut. If a knife is pulled and running away is not an option, throw anything handy at the attacker, and then run. If you cannot throw something, blitz the attacker with head strikes until he or she is unable to continue the attack.