I came up with this when I had some wire to strip and found that using a utility knife to hand slit the insulation just wasn't going to work. I needed something to hold the knife and the wire in a consistent position while I ran the blade along the length of the wire. The materials you'll need can be just about anything. To do it well you'll need several thicknesses of metal. I had some aluminum plates that were used as shims in a large format scanner. You can buy similar pieces of metal at any hardware or hobby store in many thicknesses. You will then need a cutting blade. I used the blade out of my utility knife. One of those blades that is already scored so when the tip gets dull you just snap it off and you have a new, sharp tip. You will also need some way of holding the device that can withstand a significant amount of force. I used a bench vise. You'll want to wear gloves for sure and possibly eye protection. It is possible for the end of one of your wires to fling up and smack you in the face. Using the device this way prevents you from getting cut on the blade, if you set it as I describe below. It also keeps the wire straight as it's being slit.
This is a modification of a device I made to slit plastic trim down in width. It works the same way, just doesn't cut clear through the material being cut, in this case insulated wire. You can see all of the pictures on my Flickr site. I will only have a few in this instructable.
Once you've seen the Flickr photos you can see how the setup needs to be changed a little to strip wire. The main differences are the cutting depth and angle of the blade and the depth of the slot.
In this picture I illustrate four variables. A+B equals the diameter of the wire you are trying to slit. Adding the blade between them later on will provide enough play for the wire to pull through easily. C is the blade height. For trim, the height can be big because you are cutting clean through the trim. For wire the height is very low, the thickness of the insulation you are slitting. D is the slot depth. For trim slitting the slot shoud be deep to help hold the down force device in place and to protect your hands from the blade. For wire, the slot should be slightly less than the wire diameter. This way your down force device will make contact with the wire as you pull it through the slot.