Learn to defuse violent altercations. Insults, taunts, it all means nothing, just learn to ignore them and walk away. If you have to defend yourself and you happen to be unarmed the use any object you can between you and the attacker. Umbrellas, chairs, anything you can use for defense and to keep the distance should be used.
If you have no other choice but to fight unarmed, then go for control of the arm with the knife and grappling, using and arm bar to disarm or break the attackers arm. The Army Combatives FM (pdf) explains how to do this depending on the angle of attack but in reality a knife attack is awfully unorthodox and hard to predict. Its about speed and strength. Getting control of that arm and disarming the opponent as quickly as possible.
Maybe some other good advice would be to have a small Indivdual First Aid Kit (IFAK) with you.