The thing that really a noticeable difference in my immune system was working in a bar. You come into contact with so many people, many of whom don't take good care of themselves, and end up collecting / washing the glasses they have been drinking out of, shaking their hands, handling their IDs, wrestling them out of the building. I was sick constantly for like six months, but after that, I almost never got sick again.

Sadly, it appears to be wearing off. I was sick for a fair bit last winter, and it'll probably be the same again this year.