With the weak economy and fewer business travel warriors roaming the skies, the airlines have been forced to reduce capacity either by reducing the number of flights and/or downsizing the planes. What this means is that there are fewer empty seats on each flight and more passengers are being forced to get cozy with each other. It is more important than ever to be thoughtful of your fellow passenger by observing the rules of good hygiene.
1. Eliminate as much body odor as possible. This can really be done effectively by taking showers regularly and using deodorant. Now there are some individuals who feel that a body's natural aroma should not be denied; you must be European, feel free to use European airlines such as Lufthansa or British Airways and so forth. However if you are flying on an American airline, you should follow American hygiene practices. Your American seatmates will thank you for it. Otherwise, trade seats with an American who is sitting with a European and everybody will be happier.
2. If you use perfume, cologne, after shave, eau de toilette etc. use a scent that is not overpowering and use it sparingly. Less is more in this case.
3. Control your halitosis (aka bad breath). Always have a supply of minty tasting gum or breath mints.
4. Watch what you eat before your flight; try not to have anything garlicky or any strong smelling spice. I myself love garlic. Life without garlic is a life without flavor, but I don't want people to know what I had for lunch and I'm not interested in what other people had for lunch either.
5. Before your flight, don't eat anything that will cause you gas. You'll have to expel it either through the front door or the back door, either way, it is rude. It's always a good idea to have tissues with you to discreetly burp. If you feel a fart coming, please hold it until you get to the bathroom.
Of course the above is just a starting point. Anything you want to add I'll consider to the above list.