Well, hand washing of course. I do use hand sanitizers (alcohol based) when we get in the car from shopping or whatever. I don't like them ideally but I'd rather have that than two sick kids. I wash my own hands like crazy.
Also, I do think that Xlear (xylitol) based nasal wash used once a day (or twice, three times if around a lot of other kiddos that day) can keep kids healthy.
I hate to mention it but just being around large groups of kiddos is asking for trouble at the toddler age. I still do all that stuff with my two but sometimes we do pay for it.
Keeping immunity up with making sure the child has adequate zinc. I also use colostrum for immune system building and fish oils can help with viral resistance (the vitamin A actually I think is what does it) and are good for the brain anyway.
Then I also remind myself that the immune system needs exercise too--and that is what those viruses are for! In our case, though, colds inevitably turn into ear infections for one of my guys so we avoid them as much as possible.