Antihistamines work by suppressing the production of moisture/mucous in the sinuses/ears; decongestants work by constricting the blood vessels and drying up mucous that is already there. Nasal sprays may contain either one, or both, or they may be steriods, which cause any inflammation in the nasal/sinus tissues to shrink, leaving more space for air and mucous to shift around.
Personally, I swear by Guaifenesin. I don't use antihistamines of any kind; they make me very sleepy, and my ENT says they can actually do more harm than good where ears are concerned; the eardrum is less elastic if it is dry, so it hurts more if you stretch it, and you put yourself at higher risk for a rupture. Guaifenesin is technically an expectorant, a class of decongestants that work by causing secretions to absorb water and become more liquid. This thins the gunk and lets it drain more easily under pressure. I also have Rx steriod nasal spray (Rhinocort Aqua) for situations where the congestion is severe.
You can also get Rx painkilling drops for ears, the usual brand is Auralgan. (It's OTC in Canada, btw.) These should not be used instead of prevention, because they can mask the pain of actual damage to the eardrum. Swallowing is better than chewing or just sucking to release pressure in the ears, actually. A bottle of water will keep you hydrated and help ear pressure, while gum won't do anything to help the dehydration.
If all else fails, ask the FA for cups for your ears; they will know what you need. (It's a coffee cup with paper napkins stuffed in it, and a bit of boiling water poured over them.) Hold the cups upside down over the floor first to check that there isn't too much water (burn risk), then fit them tightly over your ears. The cups will form a steam chamber, and the heat will immediately release the pressure. You can't ask for these during the descent, so ring for the FA when you are about 30 minutes out, and ask that they be brought right before the descent begins.
I'm not a Dr. either, but I have a chronic condition that causes constant fluid buildup in my ears. I actually did have an eardrum rupture on my first flight, and I pay careful attention now to be sure that it nevers happens again. BTW, if your eardrum does rupture, pain that has been excruciating will stop instantly and very abruptly, but then your ear will start ringing, and you will normally not be able to hear in that ear, or will be able to hear only very loud things. It takes a while to heal, and until it does, you will be told not to fly at all.
