The viruses that cause colds like it colder than we do. That's why they flourish in the winter. Wearing a hat makes it easier to keep you sinuses warm since your head gets warmer and the blood is carrying more heat away from the brain. Frequent hand washing is the best defense, but it's true that the whole anti-bacterial soap thing is overrated since viruses are usually the culprit. (It does help prevent secondary bacterial infections like the kind that cause sinus infections.) Viruses do succumb, however, to the power of soap and wash to wash them down the drain. ;-) This also assumes that you don't have any habits like nail biting that facilities the introduction of viruses to the mouth/throat, etc.
At least these are the latest theories that I have read (sniffle).
