I never get sick when I fly, but I used to until I really changed how I do things. Here's what I do:

* Take Emergen-C, preferably two packets a day. I take these daily regardless of my travel schedule.
* Take and eat anti-oxidants as a regular part of my health regime.
* Take Cold-Eeze (TM) brand zinc lozenges. Yes, the form of zinc is important, and this is the form that was tested and found to reduce colds.
* Wash hands frequently, and never touch my eyes or nose with my hands unless they're been scrupulously washed first.
* Wipe down all touched surfaces in my seat area, including the IFE pad, the seatbelt buckles, & the armrest. Yeah, I sometimes feel a little weird doing this, but hey, it works, and as Guy Betsy said, you should see what the wipes look like after - yuck!
* I also take a Fisherman's Friend throat lozenge after boarding and just before descent to prevent ear-popping. It helps to open the nasal/ear passages - not sure if this helps with cold-prevention, but it's part of my routine.

Hope that helps ~ Happy new year!