Lots of hand washing and no touching of the face. Also, I've become a believer in zinc supplements since about a month ago when I was coming down with something. I took a zinc tablet right when I realized the tickle in my nose was not allergies or any other irritant. The next day I felt crappy with more or less the usual cold symptoms, but the day after that, when you usually start to feel worse, I was 99% recovered. I can't say for certain if it really WAS the zinc and I will have to try it again the next time I feel something coming on, but I have never experienced anything like that. A one day cold.
Unfortunately, excessive amounts of zinc and vitamin C are apparently not good for kidney stone sufferers, so I won't be using it on a regular basis.