Excessive mucus dripping down the throat, especially at night, can be a consequence of a number of respiratory conditions, from colds to allergies to sinusitis. If there is an underlying condition, it should be treated.
My first recommendation to help bring your problem under control is to eliminate all dairy (cow's milk) foods from your diet, including prepared foods that list milk or cheese as an ingredient. The milk protein, casein, stimulates mucus production in many people and can aggravate asthma, bronchitis, sinusitis, and allergies. It may take six to eight weeks to see results, but a majority of those who make this dietary change report dramatic improvement.
You might also try nasal irrigation; that is, rinsing your nasal passages with a saltwater solution. You'll probably need to do it a few times before you feel comfortable with this procedure, but once you do, it can be a big help. Dissolve one-quarter teaspoon of salt in one cup of warm water. Pour some of the salted water into your cupped hand and inhale the liquid through one nostril at a time, keeping the other one closed with an index finger. You can inhale directly from a small cup or glass in the same way, or you can use a rubber bulb to gently squirt the solution into your nostrils. You can also get a neti pot, a ceramic container resembling a miniature Aladdin's lamp sold in health food stores. Whatever method you choose, the goal is to get enough water in through your nose so that you can spit it from your month. Do this several times through each nostril, then gently blow your nose.
Gargling with warm salt water and drinking more water can also help with postnasal drip.
Andrew Weil, M.D.