The largest study ever was on vit c and cold prevention and it was shown to do nothing. Zinc losenges were shown to do just as well as another lozenge that tasted just as bad. The reasoning behind the zing lozenge is that people think "hmm, this tastes funny so it will probably work."
I have found that rest is the most important thing for me. I never get sick until around finals time when I don't get enough sleep and I don't eat right. I take a multi vitamin, and hope that helps but I don't know.
I had a semi stuffy nose for a month at least (it would come and go, but I was mostly stuffy) and was told that a lot of people have low level sinus infections for extended periods of time. Because they aren't that serious people normally don't take anything and put up with it. I took some low level anti-biotics for one week and it cleared up wonderfully and I haven't had a problem since.