I'd say the most fundamental skill is falling. If you ride bikes, you'll fall, and if you ride in winter, you'll fall more. Get used to it. I'd say second is preparation; you need to dress very accurately for the weather or you'll overheat or freeze, you need speciality gear to deal with sub-zero conditions (anyone found a good anti-icing compound for goggles yet? I mean anti-icing, not anti-fogging, I get frozen ice on the inside and outside of goggles when it's below -10F. I've tried to stop breathing, but it hasn't worked so far.), and it's best to keep your machine in excellent mechanical condition because I'm sure changing a flat at -10F is even less fun than at 30F.