It's best to run. Even trying to kick the knife out of their hand is a mistake. If you mess up or they somehow cut your leg real bad, now you've not only made them angry, but you can't run away because your leg is messed up.
Although stupid, here's a good example of why the "disarming techniques" aren't applicable to real life situations:
http://www.youtube.com/watch?v=h_vvI26NnwE