I know there's tools you can buy for this, but I decided to design a simple wire outer insulation cutter. I'm using a lot of HolidayCoro 3-core wire and connectors for my display additions this year and I often nick the inner wire insulation using a utility or exacto knife.

This cutter has a slot that matches the shape of a regular utility knife blade, with the blade sticking up about 0.7mm, just enough to cut through most of the outer insulation. To use it you simply put the wire on a flat surface, put the cutter on top of it and roll the wire with it. A quick bend of the wire once done will split any remaining insulation and then it pulls off easily. The whole process takes 5 seconds. I also designed a cover for the cutter that you can print.