As with the majority of commercial aircraft pilots, I've experienced bouts of sinusitis nearly all of my career. We're told that the constant changing air pressure (even in pressurized airplanes) irritates the mucous membranes in the sinus cavities which, in turn results in an abundance of mucus being produced. The mucous (and underlying membranes) are perfect places for the formation of bacteria. My doctor tells me that in most cases the bacteria is controlled by the inbreathing of oxygen which kills the bacteria.
However, the swollen membranes (I'm getting to the point) form pockets where oxygen doesn't reach and suddenly the bacteria blooms, resulting in additional swollen membranes and the vicious circle only enlarges. Hence the prescription of antibiotics, such, as in my case, Augmentin. This can only go on for so lo before the antibiotic is no longer very effective.
Again, in my case, a steroidal spray (Beconase, here) is prescribed and used once in the morning and once in the evening during the onset of the symptoms. I have to say it changed my life in regards to sinusitis.
It doesn't work the same way as say Afrin or other decongestants that can only be used for 2 or 3 days at a time.
I've used Beconase for probably 20 years as soon as I recognize the onset of symptoms… maybe 2 or 3 times a year and it always it works. The only downside I find is that I'm certain my sense of smell and therefore sense of taste has lessened due to the steroid… but it's a fair trade-off in my opinion…
I do believe that during the winter months when the air (especially indoors) is very dry, that the use of a simple saline spray used 8 or 10 times a day is helpful.