My whole family has suffered with this problem and, have found two satisfactory solutions. The first which seems to be infallible, but requires more advance planning, is to buy oil of eucalyptus in a drug store and bring it and Qtips on flights. As soon as descent begins, dab a little under each nostril. Clears out the sinuses for a long enough period. Only downside is that a generous application may bother those around you. A somewhat simpler alternative is to carry a small bottle of Vick's Vaporub with you and dab a small amount under (not in) each nostril, again, just as descent begins. Not quite as effective as the oil of eucalyptus, but generally gets the job done.