Bring nasal saline spray! I know there are small enough bottles out there that can still be brought on the plane. Get the cheapest generic you can becuase they are all just salt water. If you can not find a bottle small enough to be allowed on the plane, get a travel bottle and fill it with saline from cheap contact solution. Sound gross, but snort a little out of your hand or pour on a tissue and wipe inside your nostrils.
I get a cold everytime I fly. But if I spray with saline throughout the flight, I manage to avoid the colds. The dryness allows all the little buggers to invade
I drink a ton of water too. I have not had a cold from a flight since I started the nasal spray though. A friend with deviated septum issues introduced me to the idea...she uses it every day. I use it all winter now with the drying heat that blasts in our cold winters.