1. Vitamins and elderberry syrup every morning. Occasional gelatin "flu shots" from this recipe on the Farmer's Almanac ( http://farmersalmanac.com/health/2014/09/29/gelatin-flu-shots/ ). It might not really do anything but it's pretty much an apple pie jello jiggler so they love it and it can't hurt (it's local honey, local apple cider, cinnamon sticks and fresh ginger in jello form)
2. Speaking of vitamins, in the winter I add chewable vitamin D for the kids since it's so good for flu-fighting and they aren't out getting it naturally from the sun as much as they were in the summer. Lots of fruits and veggies even though they're not as fresh and abundant as they were all summer.
3. Vigilance in hand washing, particularly before eating, even just snacks.
4. Soaking toothbrushes in boiling water for a few minutes about once a week. Also remembering to clean light switches and toilet handles.
5. Lastly but most importantly: Never, ever, EVER mention how rarely the kids get sick. Actually, forget I even wrote that.