Vitamin C. You get sick easily when you're training hard, it's like the immune system gets weakened by the hard work. I take 500mg three times a day (I don't know what that's in ounces) and I haven't had the flue since I started it. Before that I would eventually get sick whenever I was training hard for a longer period of time, and then I would be back on square one (well almost it seemed).
It's something that really made a difference for me. Most multivitamins have very little vitamin C in them!