Flying is a routine activity for millions of Americans, and raises no health considerations for the great majority of them. However, there are certain things you can do to ensure that your flight is as comfortable as possible. Changes in pressure can temporarily block the Eustachian tube, causing your ears to 'pop' or to experience a sensation of fullness. To equalize the pressure, swallow frequently; chewing gum sometimes helps. Yawning is also effective. Avoid sleeping during descent; you may not swallow often enough to keep ahead of the pressure change. If yawning or swallowing doesn't help, use the 'valsalva maneuver':

* Pinch your nostrils shut, then breathe in a mouthful of air.

* Using only your cheek and throat muscles, force air into the back of your nose as if you were trying to blow your thumb and finger off your nostrils.

* Be very gentle and blow in short successive attempts. When you hear or feel a pop in your ears, you have succeeded. Never force air from your lungs or abdomen (diaphragm); this can create pressures that are too intense.

Babies are especially troubled by these pressure changes during descent. Having them feed from a bottle or suck on a pacifier will often provide relief. Avoid flying if you have recently had abdominal, eye or oral surgery, including a root canal. The pressure changes that occur during climb and descent can result in discomfort.

If you have an upper respiratory or sinus infection, you may also experience discomfort resulting from pressure changes. Postpone your trip if possible. (Check to see if your fare has cancellation or change penalties.) A final tip on pressure changes: they cause your feet to swell. Try not to wear new or tight shoes while flying.

Alcohol and coffee both have a drying effect on the body. Airliner cabin air is relatively dry to begin with, and the combination can increase your chances of contracting a respiratory infection. If you wear contact lenses, the low cabin humidity and/or consumption of alcohol or coffee can reduce your tear volume, leading to discomfort if you don't blink often enough. Lens wearers should clean their lenses thoroughly before the flight, use lubricating eye drops during the flight, read in intervals, and take the lenses out if they nap. (This may not apply to extended wear lenses; consult your practitioner.)

If you take prescription medications, bring enough to last through your trip. Take along a copy of the prescription, or your doctor's name and telephone number, in case the medication is lost or stolen. The medicine should be in the original prescription bottle in order to avoid questions at security or Customs inspections. Carry it in a pocket or a carry-on bag; don't pack it in a checked bag, in case the bag is lost.

You can minimize the effects of jet lag in several ways:

* Get several good nights' sleep before your trip.

* Try to take a flight that arrives at night, so you can go straight to bed.

* Sleep on the plane (although not during descent).

* During the flight do isometric exercises, eat lightly, and drink little or no alcohol.

Try to use a rest room in the airport terminal before departure. On some flights the cabin

crew begins beverage service shortly after the "Fasten Seat Belts" sign is turned off, and the serving cart may block access to the lavatories.