Here is how I have done mine which has worked well for me.
1. Decide how many wires are going to be in each group.
2. Make a mark with a extra fine tip sharpy (or your choice of writing instrument) in the groove inbetween each set of wires you want to split.
3. Find a XActo (doesn't have to be XActo but I would suggest at least getting XActo blades) hobby knife with a #11 blade.
4. Lay the cable down flat on a table or some other flat sturdy surface (preferably on that can get a little scratched).
5. Place the #11 blade so that the entire sharp edge is resting in the groove that you want to cut.
Before you proceed to step 6 it is very important to make sure that step 5 has been done correctly or bye bye cable.
6. With the entire sharp edge resting in the groove apply pressure to the blade and pull up on the cable the same time.
7. Using the approximately 1" cut that this has started you can just pull the sections apart and it will follow the groove that you started your cut in.
8. Once you have made all your cuts, use spiral wrap, split loom, electrical tape, cable ties, old shoelaces, really whatever the hell you want to group all of the segments together.
I wish that I had a digital camera to take pics but I don't so my description will have to do. If you have any questions feel free to post them here or PM me. Using this method I would say I have rounded around 10-15 cables and have not FUBAR'd one yet. Even if you don't use this method I would suggest using one that involves as little moving of the knife as possible, I think this minimizes the chances of slicing a conductor.
Eric