Antihistamine won't help, and in fact may do harm, because it dries up and thickens the mucous in your sinuses. A decongestant will help, and I also recommend Guaifenesin (Robitussin or equiv. generic); it's classed as an expectorant, but what it really does is thin out and water down nasal secretions so that they can more easily drain through your Eustachian tubes. Always carry bottled water with you on a flight and drink it often; it helps a LOT, because swallowing helps open the Eustachian tubes, and planes are dehydrating anyway.
I have chronic ear/sinus problems due to Meniere's disease, and I have had my left eardrum rupture on two occasions while on an aircraft. A simple cold would almost never do it; but a raging sinus infection could. You would know if it happened to you, the pain is initially excruciating, and you would suffer a hearing loss for at least several days, probably accompanied by unceasing ringing in that ear. Eardrums do heal with proper medical treatment, so a ruptured drum does not automatically mean permanent hearing loss.
Preventative meds are your best defense, but if they don't prove effective, there is something else you can do in-flight that will help: cups for your ears.
FA's will know what you want if you ask for them. They will give you two disposable coffee cups stuffed with paper napkins, over which boiling water has been poured. First, turn the cups sideways over the floor to make sure that all the water has been absorbed by the napkins, you don't want to burn yourself. Once you're sure they won't drip, put the cups over your ears Princess-Leia-style and hold them in place as long as you can tolerate them. The steam from the water will help to equalize the pressure in your Eustachian tubes, and will almost certainly reduce or even eliminate any pain. If you anticipate that this will be an issue, ask for the cups before the actual descent, and then keep them covered to retain the steam. (This is because the FA's can't actually get them for you on final descent; you need to get them before they have to strap in.)