Zinc. Sounds simple but I read about it here on the dis and I've used it several times since and it's worked like a charm. And get plenty of sleep!
Zinc! Zinc!!

I have literally not gotten a cold in years and years. I can't remember having one at least the last 5+ years.
I start to feel that tingle coming on and start sucking zinc lozenges (2 or 3 a day) and it never progresses to a cold.
Don't worry about buying the expensive Airborne. Cold Eze or Zicam. Just find the cheapest generic zinc and it will be gone.
We also proactively take zince if we are about to go on a long plane trip or WDW and have yet to get sick.