Taking in extra liquids helps keep the sputum more fluid and therefore easier to expel. It doesn't really matter what type of liquid you drink, although tea, soup and other warm liquids may feel better than cold ones. As a bonus, warm fluids may also soothe the irritated throat that may result from all that coughing.
You can also use water for a steam treatment. Fill the sink with hot water, bend down to it, cover your head with a towel and breathe in the steam. Add a few drops of eucalyptus, peppermint or rosemary oil, if you have one of them. These help clear and soothe the respiratory passages [source: WebMD].
With all of this in mind, there really isn't definitive evidence that swamping your system with extra hydration is really best practice when you're sick. If you slug gallons of water each day, you may actually throw off your body's chemistry, making it harder — instead of easier — to recover from various diseases. And in a worst-case scenario you could trigger hyponatremia, or water intoxication, which in rare cases can actually be fatal [source: Mayo Clinic].
Your best bet is to drink when you're thirsty. And make sure that your urine is pale yellow instead of dark — too yellow and your body likely needs more moisture.