Don't train AS hard?
Serious suggestion. I used to work for a newspaper (hundreds of miles of travel a week + long days on your feet with a bunch of gear). For the 14 months I worked there, I also went to the gym 3-4 days a week before work. Nothing so strenuous that I couldn't move afterward, but enough to stay in shape. And in that year plus, I never got so much as a minor cold. Hence my suggestion to train, but just not like a total beast. Obviously everyone's different and your results may vary!