Equalize your ears early and often - as soon as the announcement happens that the descent is started - equalize your ears... and continue to do it often, don't wait for the pain to start as then it's too late, the tube from your throat to your middle ear is already pinched off.
If chewing gum or yawning doesn't help you can try the Valsalva maneuver. Close your mouth, pinch your nose and blow. Then swallow. This is commonly used by pilots during descent of aircraft or divers to equalize the pressure.
Sometimes chewing gum is not enough, especially if you are a little congested that day. Bring an empty water bottle with you though customs ( they get all agitated if you are carrying liquids, but I've never had any trouble with an empty water bottle.) Fill it up at the water fountain before you get on the plane and take frequent short sips as the plane takes off. You can ask the flight attendant for some more water during the flight for the decent.
My suggestion, since it has happened to me, if gum does not work, then block your nose completely with your fingers and mouth shut, and try to blow air out. It will unblock your ears as they are the only channel where the air can come out from. Try it.