^^^ to
wb8iny suggestion of seeing a doctor, and an ENT if possible.
Other advice (restating from above):
1)
Keep hydrated! Especially true if you're taking a decongestant, but even if not, keeping the sinuses from getting dry helps a lot! (and it's very healthy for other reasons.)
2) Take a
pseudoephedrine-based (NOT
phenylephrine-based) decongestant; 2 30mg tablets (the usual "immediate release" dose) last about 4 hours, and take about an hour to start to help. 12 hour (120mg, typically) or 24 hour (240mg, typically) sustained release ones are good too, but at least the OTC ones don't usually really last the full advertised time for me. If you're outside the US and can still get it, consider PPA (
phenylpropanolamine) - that was very good stuff before they banned it.
3) If you're at all congested, take the decongestant an hours or two before your flight, don't wait for before descent. Unless it's a very short flight or you're using sustained-release, that probably means a second dose at the 4-6 hour mark.
4) If that doesn't help, try a nasal spray decongestant. These are immediate-acting, and should
NOT be used regularly as they can cause BAD rebound congestion, f___ up your sense of smell and are very irritating. The two options are phenylephrine (same chemical as the tablets, but it works better as a nasal spray; of the two options it's shorter acting, a bit milder), and
oxymetazoline (aka "Afrin" - longer acting, stronger, but also more prone to causing all the usual problems with these.) Phenylephrine comes in 1/2% and 1% concentrations, and sometimes a 1/4% version which is mildest - I'd recommend trying this first if you can find it.
5) One other oral medication worth trying as an adjunct to a decongestant is
guaifenesin (a decongestant) - it thins out mucus both in your chest and nose/sinus, and helps you get it out. Pretty mild drug, but unlikely to help a whole lot either - still, it can't hurt. Usually sold as "cough syrup," but they do make cheap generic tablets, or expensive (but handy) sustained release ones sold as the brand name Mucinex. They also make an expensive "behind the counter" formulation Mucinex-D which combines it with pseudoephedrine. Lastly, if you can get a prescription, there are some prescription formulations of pseudoephedrine+guaifenesin. (In some places outside the US, there are some mucolytics like
Acetylcysteine available, and these might help as an alternative)
6) One other route to try would be an antihistamine (allergy medecine), if your issues are allergy based it might help. There are quite a few sedating ones available OTC, but the two big OTC ones these days are loratidine (claritin) and cetirizine (zyrtec) which are non-sedating. There are some other non-sedating ones by prescription only.
7) If you have nasal/sinus issues because of allergies, one other thing which might help is a
cromolyn nasal spray (name brand is Nasalcrom); doesn't help me but YMMV.
8) One other thing that's very effective for allergies or other persistent congestion are steroid nasal sprays; in the US these are prescription only, but I believe they may be OTC in some other countries (ignoring places like Thailand where just about everything is OTC.)