I have personally found that soups are excellent foods to emphasize during a cold, especially chinese hot and sour soup. Soups provide water, are easily digested, are high in nutrients, and hot and sour soup in particular makes you sweat. Chicken soup has also been shown to be an effective cold remedy. It contains cysteine, an amino acid which thins the mucus in the lungs, and has been shown in studies to increase air flow through the respiratory passages. Adding garlic, onions, red pepper or hot spices to your chicken soup adds to its effectiveness.
Many herbs are useful in treating colds, usually due to their immune enhancing effects.
In Chinese Medicine, a formula called Yin Chao is often used to prevent and treat colds. If used early enough, it is often successful in stopping the cold in its early stages. This formula is widely available a herb stores in Chinatown. In western herbal medicine, echinacea, goldenseal, and licorice are a few of the most commonly used herbs. These are all potent herbal enhancers of immune function, and when used properly, often shorten the length and severity of the average cold.