I eat well, lots of fruit and vegetables, but I work in a Childrens hospital emergency department and am often on shift after a long training ride which however much you wash your hands still exposes me to more bugs than most!
Read some papers on role of multivitamins with probiotics in healthy 30ish year olds, seemed to reduce number and duration of colds. Also read the stuff about zinc and vitamin c - neither very compelling. Finally good carbohydrate drinks during and post training seem to minimise drop in immune markers post excercise although whether this helps with actually getting infections is unclear. I think I will look at multivitamins with probiotics and try not to do my long rides on days when I work an 8 hour shift afterwards!