I wash my hands frequently, and I don't touch my face if I've been, say, strap hanging on one of those shuttles between terminals or handling money, or otherwise been in contact with a lot of other people's germs. Although soap and water are best, I carry a tiny bottle of hand sanitizer for instances when it's not convenient to wash before meals (such as before meal service in Y), although I prefer not to overuse it for the reasons mentioned above.
I also take vitamins and minerals, especially C and zinc, and get plenty of sleep if at all possible. If there's no particular social obligation in the evening, I'll just go to bed early with a good book.
At the first sign of a scratchy throat or runny nose (that isn't allergy-caused), I pop extra zinc and go to an Asian restaurant for some spicy soup. Usually, the symptoms either disappear or develop into only a mild form of whatever bug is going around.
I haven't had a knockdown drag-out totally incapacitating case of the flu since December 1999 (which I remember well, because I spent Christmas Eve and Christmas Day unable to do anything but listen to classical music on the radio as I drifted in and out of sleep.)