Like they said, lots of water, lots of good whole foods, and I'd recommend a good vitiman/mineral if you aren't already taking one (because they are probably depleted in your system right now). Also, I always take a lot of vitiman C when I have colds or flu's. I take about 5-8 grams of it when I'm starting to feel sick and continue to do so until I heal. I've cut down my sick time in half by doing all of this stuff.
(I wouldn't recommend the caffiene advice however, because your body may be drained for good reason and working out harder because you have that artificial boost could lead to more sickness. Also, because with a flu you generally lose a ton of fluids, and caffiene is a natural dieruetic, you could get much more sick with more loss of fluids.)