Put a piece of chewing gum in your mouth and chew it vigorously, swallowing the resulting saliva as often as possible. The act of swallowing frequently is often enough to clear your ears.
Drink water or another drink in large gulps if the chewing gum did not help. This might help you swallow harder than the chewing gum alone, which could make the difference between almost clearing your ears and successfully relieving the pressure.
Yawn as wide and hard as you can if drinking was unsuccessful. Do this several times in quick succession.
Pinch your nose if nothing else has worked. Holding your nose shut completely and keeping your mouth tightly closed, try to exhale through your nose. Do not do this too hard, as you may end up hurting yourself.