You want to support your immune system. So healthy food, vitamin C (some say vitamin D helps, too), echinacea... I really like chopped up garlic in nice raw honey, BUT that gives me heinous heartburn so I can't do it often. When I do, it tends to stop the cold flat.
If you have a Korean restaurant (or cook) available, there's a spicy bean paste soup they make that has fresh bean sprouts added at the last minute, and that is really tremendous for helping the immune system. When DH is around (he travels for work) and can get the ingredients, he likes to make it when someone is sick. His mom argues that it doesn't do any good without the marrow in it, but we're vegetarian and it has worked just fine without that for us.