The goal is to keep the wind off your fingers, ears and toes. Cover your ears with a winter sports band. They're available in any outdoors store, and fit well under most helmets. Choose layered gloves. My personal choice is an inexpensive pair of Thinsulates.
Toes can be a problem.
The best solution I've found is Foot Warmers. These are one-use paper packets the contain a mixture of iron filings and other substances. When exposed to air, they heat up. They're just the right size to slip into the front of a shoe, and will keep your toes toasty for up to six hours. But if you ride regularly in the winter, you're going to encounter slick patches.