Water, water, water!
Your body needs to do more than kill germs; it needs to get rid of little germ corpses, and germ waste products, and little baby germs (metaphorically speaking).
Your body has a fantastic system for getting rid of things it doesn't want, but its effectiveness is directly related to the amount of water it gets to use. When I feel something coming on (or when I'm not feeling sick but everyone else in my house is, like now) I aim for 8oz of water per hour.
First signs of the flu showed up in the house on Monday; so far, so good.