Understand the danger presented by a knife attacker. Have a friend attack you with a magic marker and try to disarm him while avoiding being marked by the ink. Try this exercise with several different training partners. You will be surprised at how difficult it is to avoid being marked.
Train frequently, and at full speed when possible. Half-speed training is useful for learning techniques, but does not prepare you for a full speed attack. Wear protective clothing and eyewear, and have your training partner attack at full strength.
Train against the most common forms of knife attack: the forward thrust, slash, downward stab and upward thrust. Practice with multiple training partners. Learn to defend against and disarm attacks using the forward grip and the reverse grip.
Evaluate and use your environment. You may have an object available to block the knife, or to use as an offensive weapon. Look for something easy to grip.
Step away from the line of attack and protect your vital areas. Avoiding the knife is your first order of business. Do not get cut. Be aware that the attacker is likely to misdirect your attention with fakes. As you avoid attack, maintain a balanced posture to facilitate your disarm.
Control the knife hand. Maintain your balanced posture and look for an opportunity to grab hold of the hand or wrist. Hold onto the knife hand or wrist, and do not let go until the attacker has been disarmed. Be aware that the attacker can still attack with his free hand, or with kicks, elbows and knees.
Deliver a blow to a vital area, if possible. Once you control the knife hand, attack the most available sensitive area. Look to strike the groin, throat or face. Gouge the eyes, deliver a head butt, bite the knife hand or break the thumb. Remember that you are now engaged with someone who is attacking you with lethal intent. Use every tool at your disposal with full force. Above all, maintain control of the knife hand.
Disarm the attacker at the first opportunity. This can occur directly after you control the knife hand. Generally, look to force the knife out of the attacker's hand toward the thumb and wrist. Practice the actual disarm techniques frequently during training to learn the sweet spot for disarming an opponent. Be sure to have your practice partner use different knife grips and attack techniques.
Gain control of the knife. Your situation improves considerably if you hold the knife and your attacker does not. Look for a way to exit the confrontation.