In addition to decongestants, a very simple thing you should investigate is a "saline nasal spray". You can "irrigate" your sinuses and other internal passages and it does help, particularly for me on airplanes. It's not a drug, so you can do it as often as you like. Just be careful not to squeeze too hard on the bottle or you'll have salty stuff going down the back of your throat. Yuk.

A doctor might have other drugs and such to offer you. There are some particularly powerful (and nasty tasting) spray antihistamines. I save mine for only the very worst of allergy attacks. There are also a number of nasal "steroid sprays" (I use Nasonex; Flonase and Rhinocourt Aqua are medically similar). These reduce inflamation and thus leave more room for all that fluid to drain from inside your ears.

Of course, I am not a medical doctor, but I've learned too much about this sort of thing dealing with my allergies...