I travel an awful lot, and for a while I used to come down with a cold fairly frequently after traveling (one of the usual hazards of air travel).
Somewhere along the way -- a few years back -- someone suggested that I try Airborne, the tablets you drop into some water that fizz up and put a bunch of random vitamins and whatnot into your system.
From the beginning, I was pretty skeptical about it (I don't tend to believe in such things), but figured I didn't have much to lose, and started taking Airborne when I travel.