My kids didn't get sick at all last year. there were several bouts of flu/gastro going around and they never got any of them. DS2 did get a tummy bug for a day, but that was during school holidays so he hadn't even been to preschool. Fingers crossed it happens like that again this year. I don't do anything special all the time - they just eat and sleep well. I used to give probiotics to boost them if they were starting to get a bit run down but now i don't worry.