Here's the answer given in Timberwoof's Motorcycle FAQ:
In many places, it gets too cold or snowy to regularly ride
during the winter, so you have to lay the bike up for storage.
Check your bike's user manual for how to do this.
In the San Francisco Bay Area, 'winterizing' a motorcycle means
packing your rain gear in the saddlebags and velcroing closed the
vents on your Aerostich.
Cold weather riding can be done, as long as their isn't snow or ice
on the road. Most people who enjoy this form of torture purchase
plug-in heated accosories, such as a heated vest, and gloves or
heated handlebar grips. A lot also install wind screens on their
bikes, in order to cut down on the windblast. You have to be
carefull though, because your tire's rubber compound doesn't warm
up as much, and that will cause less traction.
An old "Biker" taught me to stuff newspaper in the front of my
Jacket and Chaps to help isolate the cold blast of wind away from
your skin. I have also used those little hand warmer packets that
you can get from some retail outlets and gas stations, I have used
them in my boots and in my gloves, because I'm too tough or too
stupid and cheap to buy all the heated clothes and handle-grips.I
live and ride in the "Great White North" and may leave when the
weather is nice, but ya never know what type of weather you'll be
ridin' in 100 miles down the road. Ride safe, if you get too cold,
Keep riding. There are a bunch of us who have our own forum just
for riders who ride year round, we have members everywhere from
Alaska to Florida and plenty of state in between.