Yeah, a glass of orange juice will usually yield you a full serving of vitamin C. When I was running in high school/college we were always reminded that constant exercise (combined with the correct clothing for the weather), vitamin C, and a decent diet kept us race ready. It works, but even with that your still at risk to getting sick every once in a while. The key is to treat it quickly, complete treatments (if your taking something other than OTC medications), and make sure you rest the body. A few days off can prevent a simple cold from turning into something much worse.
Ask 100 different people how to stay free from colds and you'll get 100 different answers...most will contain exercise and vitamin c though.