SLF here, a trick that works for me when one ear is blocked (usually the case), is to pinch the nose, and the unblocked ear with a finger and then ''blow''.
Dimitris,
That's the valsalva. Plugging your ear, however, won't make a difference, as there's no air passage through the ear, to block with your fingers. You can plug your nose and pressurize your sinus cavities and the eustachian tubes by "blowing," but plugging your ears shouldn't make any difference.
Don't rely on that technique to relieve a pressure imbalance, however. If you find yourself blocked, or your sinuses plugged or with ear pain, don't fly...the passage between the sinus and your airways, and the eustachian tubees are very, very small. They can easily become occluded when you're ill, and it's very possible to get to a point in flight where you can't relieve the pain. A blockage can become incredibly painful, and can cause separation of the lining of the sinus cavities as well as eardrum rupture.
The solution, of course, is to avoid flying if you're ill, or supsect you might be so (even if you don't feel it yet). Better safe, than sorry.