Echoing what Sqad says, for the short term, OTC nasal sprays used for de-congestant, work pretty well... But avoid long term useage (anything more than about a week) since you cna become somewaht addicted to them. Not in the usual sense of addiction, but the relief time becomes shorter and shorter and you'll soon find the need to apply them within 2 or 3 hours each time.
There are prescription obtained cortisone nasal sprays that are only applied once per day. They work well but don't give the immediate relief experienced by the OTC types. One problem I've experienced (almost every pilot I've known has had bouts of sinus infection brought on by irritation of the sinus mucous lining due to constant change in air pressure) is a noticeable loss of sense of smell... not disastrous but just noticeable...