The hand washing has made the biggest difference for me. I used to get sick after approx 1 in 4 flights, now hardly at all. i have some hand sanitizer but try not to touch food at all with my bare hands unless at home
It's amazing how often in public you see people munch sandwiches with their bare hands and other food as well. Yesterday in the SAS lounge i saw somebody pick up a slice of cheese with their bare hands and fold it up and eat it (just the cheese, no bread).

On a similar topic I avoid food in shops/hotels/lounges that people can have touched. People often ignore the tongs provided (eg biscuits in the big glass jar) and plunge in with bare hands. This varies by country. Visit Lidl in the uk and half the customers take the bread rolls and pastries with their bare hands. Customers in Germany and Finland are using the tongs.