Active sparring is the best of all knife drills, provided one spars against a variety of opponents.  (If you always spar the same person, you'll develop bad habits, because you'll be prepared to spar only that particular person.)  In active sparring, two (or more) individuals armed with soft rubber training knives or marking blades duel each other.  Such a drill develops reflexes, speed, flow, timing, and the proper mindset for knife fighting.

Active sparring is itself a drill, rather than a category of drills.  All you need is a willing partner against whom to test your skills.  

Disarms and defenses are those drills that involve "defanging the snake" or simply deflecting an incoming knife attack.  Rigid training knives are best for this because many disarms involve using leverage to take or trap the knife.  A flexible "knife" would be unsuitable for such training.  Training disarms and defenses requires the willing partner to "attack" with a knife -- at slow speed at first -- while the defender practices deflecting, trapping, and taking the "blade."  Special care must be taken to avoid injuring the training partner doing drills of this type, as it is very easy to harm someone's joints in this fashion.

The drills will vary depending on the techniques used (specific knife defense techniques are beyond the scope of this article) and the style in which one trains.   

Disarms and defenses are best trained under qualified instructor supervision.  Be aware that a rigid training knife is potentially dangerous, especially if thrust into an eye or the throat.  While training disarms and defenses it is very easy to get into bad habits, hurt the "attacker," or experience injury as the "defender."  Wear protective gear as appropriate.

Drills for training forms and mechanics can be as simple as watching yourself in the mirror or watching your own shadow on the wall as you move through the mechanics and flow of your particular fighting style.  Aluminum trainer blades are best for this purpose, as they are closest to live blades (and thus help you develop the appropriate mindset for wielding the knife) but still relatively safe if you make a mistake.

HANGING OBJECT TARGETING AND CUTTING

Hanging object targeting can be done with training knives or with live blades.    Using tennis balls, ping pong balls, sheets of paper, pieces of cardboard, or other suitable targets, hang the targets from a wall or ceiling with cord or string.  Using your knife or trainer, practice stabbing and slashing at the targets to get the feel for hitting where you're aiming.  For best results, hang targets at different heights.

Always use extreme caution when test-cutting with a live blade!  I once stabbed myself in the arm performing a swinging cut with a double-edged dagger that missed its target and landed in the inside of my forearm.  One very useful piece of equipment is a simple pair of leather wrist cuffs.  Remember that your knife can maim and even kill you.  Respect it.

Wrist cuff obtained from Combat Technologies.

HANGING BOTTLE TEST CUTTING

Alliance Martial Arts lists some useful drills for knife and self-defense training.  Pete Kautz of Alliance wrote a very good article for Knifeforums Magazine in which he described test cutting using hanging plastic bottles.  This is a good drill and one in which anyone could engage.  Targets can be empty or full of water (with predictably messy results).  This is really just a variation on the hanging object targeting drills, the stipulation being that the targets are bottles.  Do not use glass!

I like to use heavy jugs such as those in which Arizona Iced Tea is packaged.  The jug below was sliced by a Cold Steel Tai Pan while hanging from the ceiling.

Arizona Iced Tea jug (very thick plastic) slashed with Cold Steel Tai Pan.

PURCHASED DUMMY TARGETING

More than one human-shaped punching "bag" exists on the market.  Buying one of these is a quick and easy way to practice accurate targeting against a person-sized opponent.  Training knives are used for this purpose, obviously, because live blades would carve up and destroy the dummy.

DUMMY TEST CUTTING

There are a number of different ways to construct a test-cutting dummy.  The most common methods involve some type of hard core -- PVC or wood, for example -- covered in padding of foam or layers of cloth.  Duct tape is usually used to wrap the padding (and can be used to extend the life of the cutting dummy).  The object is to create something that provides resistance but which can be cut from the outside to a harder core (which simulates bone).  This gives the knife wielder some idea of what to expect when striking a human target.  The design of the dummy is really up to you.  Use your imagination and experiment with different methods.

CUTTING MEAT

Cutting slabs of meat is another way to get some idea of the resistance one's blade will encounter against a human target.  (I don't have to tell you where to go obtain meat --  I think you can figure that part out yourself.)  Slashing into a slab of beef will also impress on you the damage a sharp knife can do to the human body with very little effort.

CAUTIONS

Remember that the purpose of training drills is to develop the skills necessary for wielding a knife effectively for self-defense.  Practicing a lot of flashy, useless maneuvers that look impressive but have no application is a waste of your time and may cause you to do something foolish in the stress of a real encounter.  When performing these drills and in devising drills of your own, always ask yourself a few questions:

• What skill does this drill develop? 
• Will the drill create bad habits as performed?
• What is the application of the skill developed?
• Is there a way to develop this skill more efficiently?

Never train when you are tired, either physically or mentally. 

Use your head and stay safe while learning what you must learn to protect yourself.