The NY Times has a regular section that aims to debunk common health-related myths. This week, they take on the claim: Can Hot Liquids Can Ease Symptoms of a Cold or Flu? Unfortunately, there isn't a lot of research examining these effects. However, the research that is out there supports hot liquids over room temperature or cold ones. (See the study referenced here and I searched for any meta-analysis studies and could only find this one study.)
Of course, there are still lots of questions out there regarding how much liquid you should drink, does caffeine affect these results, is there an ideal beverage to drink over others, etc?
So the next time you have a cold or the flu, consider consuming hot liquids over cold ones. Your nasal passages and respiratory tract will thank you for it. Also should you need to visit your health care provider, be sure to tell them all of the self-care steps you are taking to ease the symptoms. They will thank you for it too!