Hot tea, and lots of it. Ideally a kind with "spice" in its name or description - spiced chai, Constant Comment, etc., the ones that are full of cinnamon and its ilk. (I don't know what the other spices are, and the tea people usually don't tell us.) These all come in decaf if the quantities involved will be problematic at full caffeinated strength.

The important thing is to keep the lungs and sinuses as steamed-up as possible, for minimum discomfort and maximum loosening-up of the crud. Chicken soup does this and also replaces electrolytes (yay, sodium), but one can't swallow soup all day long.

Hot showers as well, and a humidifier in the bedroom isn't a bad idea.