I'm not sure you how has been said about this, but I thought I could throw my two cents in. I recently had to attend a meeting at work about nutrition promotion. Our nutritionist at the hospital actually brought up some valid points.

1) Exercise - Even a 30 minute walk is sufficient.
2) Multivitamins
3) Fresh fruits and vegetables
4) Don't smoke
5) Don't eat bad food, a dessert here and there isn't going to kill anyone
6) Low calories with optimal nutrition. Basically from what I gathered, it takes the body more energy to rid the body of waste and bad nutrition that cannot be used to boost the immune system. She was much more detailed, you probably get the point.
7) Protein, Protein, Protein. She really stressed this and this is what I wanted to throw in. She said to think of your body as one giant muscle, muscles cannot grow without protein and neither can your immune system. She said to think of grams of protein as power houses for the immune system to build new components.
Plent of sleep, basically this is when your immune system does its work. This is my huge flaw, my anxiety causes me to get about 3 hours of sleep. This leads to increased stress with decreased ability of the body to deal with stress. It turns into a bad cycle.

Hope this helps. -Colt1911