Travelers will soon fill the air and many will experience ear pain, sometimes severe, during the flight.

It is caused by a condition called eustachian tube dysfunction. The eustachian tube connects the air filled middle ear space to the back of the nose allowing pressure between the two to equilibrate.

Swelling in your nose, as is caused by colds and allergies, can block the flow of air through the eustachian tube. When you fly, the air in your middle ear space expands with ascent, and shrinks with descent. If the air does not clear well through the Eustachian tube, it can cause an ear block which is quite painful.  It can also cause a sinus block, but this is less common. 

I suggest this. First, make sure you adequately treat any allergies you may have.  Second, if you tolerate oral decongestants such as Sudafed, take two about 1-2 hours before your flight. Also, pick up an over-the-counter decongestant nasal spray such as Afrin and use it about 30 minutes before you board the plane. The combination of these will minimize the swelling in your nose as much as possible to help you clear the eustachian tube.

The ascent does not usually cause ear or sinus blocks, the descent does. When you start to feel pressure building, try to clear your ears in whatever way works best for you such as swallowing, chewing gum, or performing the valsalva maneuver.

An emergency maneuver to try if you cannot clear the pain is to fill a cup with water as warm as tolerable, cover it with cellophane, and cup it over your ear. The heat makes the air in your middle ear space expand and can help clear the block.