Your middle ear is a sealed chamber containing tiny bones that transmit sounds to the nerves in your inner ear. When the pressure outside your body changes (like when you drive up a high mountain, or like when the cabin pressure in an airplane changes), a valve in your eustachian tubes opens, and the pressure is equalized. Many people reccomend swallowing or blowing while holding your nose to clear your ears if you have trouble...That's because your sinuses are connected to your eustachian tube. When your sinuses are blocked, or irritated, your eustachian tubes can become blocked as well. This is why sinus medication can help you clear your ears. I promise this is true; it is not that I don't know my nose from my ear! Antihistamines (Chlophenarimine, Entex, ternafdine (Seldane) )will help, and you should take these with you if you have to fly with a cold or allergies.
If you have a lot of trouble clearing your ears all the time, try this: