Try some echinacea (if you believe in it).
The data about this naturally occurring plant extract's ability to prevent colds is rather dubious and how it may fight illness is not understood, but one larger study showed that people with the common cold treated with echinacea had a 10 percent decrease in symptom severity and a half-day reduction in illness duration. It can be taken as a tea or as an herbal supplement, both of which have been shown to be safe and without side effects. Whether it's the echinacea that limits the cold or simply a placebo effect is still a matter of debate. A 2011 study from the University of Wisconsin found that the severity and duration of the common cold is decreased equally in people who took either echinacea or placebo pills compared to subjects who took nothing.