1) Practice "respiratory etiquette." It's the new catchphrase used by hospitals and other professional healthcare facilities, and it's pretty much an updated version of the old common sense rules: Wash your hands for at least 20 seconds with warm soap and water, especially before eating, etc., plus interesting new info about disinfecting surfaces (cold and flu germs can live for HOURS outside the body) and be sure not to share ANYTHING, even towels, with someone who's sick.
2) Gargle with warm salt water. People who gargle with water a few times a day are one-third less likely to catch colds; salt and warm water once you're already sick helps clear the virus out of your throat.
3) Drink hot tea with honey and lemon. Again, old common sense wisdom backed up by modern science -- hot drinks "stimulate digestion, ease cold and flu symptoms, and give your immune system an extra kick," honey (particularly darker varieties) has such strong antimicrobial properties it's been shown to reduce risk of flu infection, and lemon adds Vitamin C and antioxidants.
4) Use a nasal spray or neti pot. These work the same way gargling does.
5) Try brewer's yeast. Apparently sprinkling a teaspoon of brewer's yeast on your food every day can really boost the heck out of your immune system.