Number one: get adequate rest. If you're skipping on the zzz's, you're depleting your body's defenses and weakening your immune system. Not only is sleep important for preventing a cold and the flu, but it's also crucial in your recovery should you end up getting sick. Get at least eight hours whenever possible.
Already sick? Double down on the sleep. Get as much rest as humanly possible. Seriously, give yourself a break and sleep it off. And don't feel guilty while you're at it. Take this illness as your body's cry for help (and a break).