I've found that using the tiny nippy cutters by Klein (designed mostly for electronic work) does good results.
I start from the cut end, working the point of the blade between two conductors and start snipping my way straight down the cord, pausing every couple of inches and separating the plastic jacket as I go along. It makes it easier if you do this cutting at two spots so you're ending with two ribbons of jacket.
I've never nicked or cut the conductors' insulation this way. It takes some practice but I've gotten pretty good at it. Take your time.
What Joe and JCooper said. This is how you strip any round cord and many cables. Score it with a knife, bend it back and forth and pull it off. This results in a nice clean jacket cut. There are several types of strippers on the market that will do the scoring for you but with a little practice you should have no problem with a knife.