While the human ear appears to be an open canal, there are tubes inside (eustachian tubes), that not only manage the draining of fluids, but also the flow of air, balancing the pressure in and outside of the ear. When that balance is off, we feel a pressure sensation, which is relieved by "popping" the ears. Popping the ear is really a release of pressure and there are several ways to assist that release when the pressure is uncomfortable or painful. The imbalance and pressure build-up can be attributed to a change in altitude, a cold or other illness.
Swallow some fluid or make the swallowing motion. Take a small sip of liquid or if none is available, simply make the swallowing motion with your own saliva. Repeat until you feel the pressure release. This simplest remedy should be first in the line of defence when pressure is building to discomfort. Swallowing may relieve the pressure by opening the eustachian tubes so that air or fluid can escape into the middle of the ear and relieve the pressure. Typically, this remedy works in aeroplanes when you can feel the pressure building. Catching it early and relieving it will prevent it from becoming uncomfortable.
Yawn, making the yawning motion if not an actual yawn. Repeat until you feel the release of pressure. Imitating the entire yawning motion engages the inner ear and typically will stimulate an actual yawn. Even if an actual yawn is not stimulated, the inner ear may be engaged and pressure will be relieved. This is also a simple remedy and should also be initiated when pressure begins to build. This method may be employed on aeroplanes or even on a drive up a steep mountain.
Chew some gum, which will not only produce saliva, but create a swallowing motion. Ideally, the pressure can be felt when building and chewing gum can be employed to ease the pressure while it is building, preventing it from becoming painful.
Take a decongestant. If the pressure is from a cold or other illness, this method will take some time, but should still provide some relief. If the pressure is from altitude, use this method as a preventive method. Some individuals have a harsher reaction to change in altitude than others and will always experience a build-up of pressure upon the actual descent from the increased altitude. Take a decongestant prior to a scheduled flight to give the medication some time to work and help prevent the build-up of pressure.
There are more drastic measures, including pinching the nostrils and blowing, but this method does not always work and can actually exacerbate the problem.