First, face your attacker and keep the weapon in your sight, but do not succumb to weapon focus -- the tendency to focus on the weapon and lose sight of the rest of the assailant. While you are watching the knife, the attacker may make a move that catches you off-guard. And call for help!
Try to keep out of the weapon's effective range. Most often, knives used in these attacks are small and concealable, with a range no more than 4 or 5 inches beyond the attacker's reach. Attackers also often attack with slashes, so keep this in mind when gauging distance and movement. If you stay out of arm's reach from the attacker, he most likely will step in to attack, so don't back yourself up against a wall or other object, and be ready to jump back out of reach when the attacker steps in.
Use an object such as a briefcase to block the attack, and even to knock the weapon from the attacker's hand. If your fingers wrap around the object (such as a padfolio or desk tray), grab the object at one end and use the other end against the attacker -- this will help prevent your fingers from being cut.
Try to keep the attacker at bay until help arrives, and keep calling for help until it gets there or the attacker runs away. Do not turn and run unless the attacker is incapacitated, as he may catch up to you and stab you from behind.