Drink, Drink, Drink
This doesn't mean alcoholic beverages, of course. But drinking plenty of any other nonalcoholic, decaffeinated liquid (caffeine and alcohol both act as diuretics, which actually increase fluid loss) will help keep you hydrated and will also thin mucous secretions. The flu can cause a loss of appetite, but patients often find warm, salty broth agreeable. If you're not eating much, juices are a good choice, too, since they provide nutrients you may be missing.