I seem to have a rather robust immune system and don't get sick much from traveling, but other people who are not so lucky may want to check out these airplane headrest protectors from TravelKleen. They sent me a sample to check out before my last flight, so I gave them a whirl and they deliver as promised.
The idea is airplane seats don't get cleaned very well or very often, especially the overworked ones that are hitting three or four gates in a day. So you whip out one of these, attach the velcro, and put your head on something where you know the origin of it. Two come in a package and they're reusable--there's a set of plastic pieces that keeps the velcro from sticking to everything else when you fold it up. You paranoid or extra-sensitive types can also use them in movie theaters, on trains, or wherever else you're worried about catching cooties. (Just remember the old adage, "That which does not kill you makes you stronger.")
The TravelKleen headrest protectors weigh about as much as a few sheets of paper and take up almost no room. So if you're concerned about the state of where you're nodding off (or are protecting the rest of us from the gobs of mousse in your hair), check your favorite travel gear store or go to TravelKleen.com.