I definitely concur with Woody: Escrima, Arnis, and the Filipino Martial Arts in general have broken down knife combatives into a science. Your best bet would be to locate an instructor affiliated with Guru Dan Inosanto, Tuhon Leo Gaje, GM T. Illustrisimo, LAMECO, Bakbakan, Bahala Na/GM Leo Giron Arnis, among others.
Remember, despite George Hill's comment regarding guns, having a firearm does not necessarily preclude learning defensive edged weapons and empty hand skills. Proper use of force techniques are very important in surviving a knife encounter, even if you are armed. An attacker can clear over 21 feet faster than you (or even, Giles Stock) can draw and fire, so even if you get a shot off, you may be stabbed, sliced, and/or killed.
For truly awesome knife training, be sure to check out the Edged Weapons Conference we are holding on March 3-5, 2000. Top names from recognized organizations (Inosanto Academy, Gunsite Training Center, Dog Brothers Martial Arts, GSGI, Emerson Knives, among many others) will all be present to discuss, teach, and demonstrate strategies and techniques to help everyone come away with *real* knife fighting skills and the tools to further develop them. For more info, go to http://www.streetpro.com/ewc2000