How do I boost my immune system?

My family and I have been sick all month. Everything just cycles through each member, then when we've finally all gotten over it, my daughter picks up something else at preschool. We eat well, we're active, we don't smoke or drink, and we wash our hands so much I have to buy soap by the gallon. I asked my doctor what else I can do and she basically said "nothing". I feel like there's got to be something else. Any ideas?