Your body is mostly made up of water, so it makes sense that swilling plenty of the stuff every day keeps all of your cells working optimally. Drinking enough water is a common-sense defense against postnasal drip. It keeps your mucus thin and your body, including your nasal passages, well hydrated.
The old rule of thumb was that you should drink at least eight 8-ounce (236-milliliter) glasses of water a day. That's a very rough baseline that really doesn't serve much of a purpose, because some people — particularly those who or active, ill or breastfeeding — often need much more fluid.
The National Academies of Sciences, Engineering, and Medicine estimates that men need about 15.5 cups (3.6 liters or 124 fluid ounces) of water per day. For women, the number is closer to 11.5 cups. About a fifth of your hydration often comes from water in your foods [source: Mayo Clinic]. Chicken soup is enjoyable at any time, but especially when you're sick — it will add fluids and food to your body, while steam clears out your blocked nostrils.
As a rule of thumb, simply drink water when you're thirsty. And if your urine is dark yellow, it's a sign that you're not getting enough hydration. Shoot for urine that's a pale straw color or colorless instead.