Get More Sleep
I'm going to do some horn tooting, I haven't had a cold since January 2010. That's right. 3 years, cold free. What's the trick? It's hard to pinpoint, but the combination of GREAT sleep habits, oil of Oregano and keeping stress to a minimum.
That's it... that's all there is to it. Get 8+ hours of sleep and take a few drops of OoO when you are feeling even a TINY bit off.