It should go without saying but you need warmer clothes. Dress in layers, beginning with a base layer of moisture-wicking material and ending with some type of insulated outer layer. Bear in mind it will get colder still when the sun goes down so be sure and pack extra clothes. Wearing or packing a waterproof layer is also a good idea. Hypothermia (exposure) is the greatest killer in the outdoors, and is usually the result of being both cold and wet.
A survival kit is something you should always pack, regardless of the season. In the summer it will make an unexpected night in the woods more comfortable. In the winter, it could mean the difference between life and death. At a bare minimum, your kit should contain the following items: compass, whistle or other signaling device, waterproof matches, butane lighter or other reliable fire-starter, multi-tool knife, reflective (Mylar) Space Blanket, food, water and any personal prescription medications.
Because you’re riding, and weight is less of a concern, you may also want to consider some more extensive safety equipment such as: a tarp or bivy sack for shelter, a first aid kit, a gallon of drinking water, canned food, MREs or high-energy foods such as chocolate or emergency food bars, flares, folding saw and axe or hatchet.
Change your oil and filters. Breaking down now is a lot more inconvenient. Besides, this will give you an opportunity to change your oil to a winter formula for better operation in colder climates, proper engine start and less wear on your machine.
Check your battery to ensure it is fully charged. Cold weather can drain valuable “juice” quickly.
Keep out moisture. Water in your fuel, even in small amounts, is a problem any time of year. Above freezing it effects performance and starting. In winter, it freezes, blocking fuel lines and carburetion. If possible avoid any fuels containing ethanol as the water quickly separates out and freezes. Use a fuel conditioner. Also try to keep the tank full to prevent condensation.
Accessories - There’s almost no end to the list of accessories you add to make your winter riding safer and more comfortable. Anything that breaks the wind will protect you from the elements. Hand warmers are also nice amenities. Also consider a padded, waterproof seat cushion.