I have a great deal of experience here, alas.

One warning: do NOT ever take anti-histamines (Claritin for instance) when flying. These CONSTRICT the sinuses and can result in terrible pain.

Sudafed opens up the sinuses. However, it has a bounce-back effect. That is, after it wears off the sinuses often constrict more than before for awhile. If you need to take pseudephrine (generic Sudafed), you may want to take it about an hour from landing. It is the descent that gives you the trouble, not the ascent, so you don't generally need it in flight. The exception is if you have a cold, in which case you may want to take it just before boarding, and also some more on descent if it's a long (transcon for instance) flight.