Just in Case...
If renting a car, check to see if a snow brush/ice scraper is in the car - before you leave the lot. Just because you may not need it when you pick the car up, doesn't mean you won't need it before you return the car.
A pair of rubber overshoes/galoshes can be handy. They don't take up too much space in a briefcase or big coat pocket and can be real handy in keeping your feet dry (and warm).
Tip #8 is a biggie if you're traveling to the Pacific NW {WA & OR}.
Don't assume public transportation (buses & trains) will be running on-time. Just as with airlines, these can be affected by winter storms. Give yourself extra time if your using one of these to connect from the airport to downtown.
