This is the most obvious consideration when you're planning to ride in cold weather, and it also has the biggest impact on your cycling enjoyment. Layering clothing is the most effective way to combat cold and wet conditions and rapidly changing temperatures.
Three layers tend to work best in winter conditions: Start with a snug base layer made from a wicking material, such as polypropylene, wool, or silk. This layer will help move moisture away from your skin quickly to reduce the evaporative cooling effect that sweating has on your body.
Your second layer is your insulator; it should be slightly looser than your base layer to trap air near the skin. This trapped air is warmed by your body and remains there to help keep you warm. The insulating layer can be made of a variety of materials depending on weather conditions: synthetics, wool, fleece and down all work well.
Finally, choose an outer shell that incorporates a wind-blocking and water-resistant material. This final layer should be made of a Gore-Tex or Windstopper-type material that's both impervious to wind and exterior moisture, but still allows perspiration to escape to keep you dry.
Your extremities regulate temperature poorly, which is why hands and feet are often the first to get cold; thus, they need some special attention. A variety of different thicknesses and types of gloves, booties, toe-covers and arm and leg warmers may be necessary to accommodate all weather conditions. As you know, an uncovered head is a big source of heat loss. Wear an insulated skullcap underneath your helmet.