I've only just begun to take Vitamin D and mineral supplements this year. Hopefully that'll help my immune system this winter season. I recently got my annual flu shot as well, though I know that won't necessarily cancel out the possibility of getting a cold. The other thing is that I ensure my hair is dry when I go outside, especially if it's near freezing or below. If it's raining or snowing, I try to stay dry and get out of the cold as quick as possible so I don't get wet.