Have you tried emergen-c? It's that OTC stuff that supposedly boosts your immune system. I've never tried that specific brand, but I've tried others and they worked. It wasn't an instant cure or anything, but I felt a lot better.
when I've been sick, I've just rested it off and drank a lot of liquids -- hot teas help for colds, not sure what's the best for flu...
Maxie's got a good suggestion with Emergen-C. It's uber packed with stuff... especially vitamin C. It was really popular at my college. (If you get really bored, dump a packet, or just a half, on your tongue. ... I had smart friends.) But staying hydrated with the proper nutrients is important. So juices or packets like these are nice. I wouldn't go with sports drinks because of the sugar.
But yeah. Rest and liquids. If things don't get better, see a doctor... or just talk with a pharmacist.