First, the solutions to prevention: keep the humidity in the house between 50% and 55% year-round (though some specialists recommend 45%- to 50%, I've found that higher is better). Second, use a Neti pot (or something similar) to flush the sinuses when necessary (I'll elaborate on "necessary" in a moment). I've been using the Neti for four years now and, with that, have been able to reduce the number of sinusitis incidents, but it still bedeviled me until I installed enough humidifiers in my house to maintain proper humidity levels. (I use two Vicks humidifiers sold at Bed, Bath & Beyond. Each humidifier is at each end of the house.)
You can accurately measure relative humidity with a hygrometer that's accurate to within 5%. Here's the one I bought and am very satisfied with. By watching your hygrometer regularly, you can adjust your humidifiers' output accordingly for while your asleep by how much you expect your heater or A/C will turn on during the night and dry the air. I've gotten to where it's almost a fun challenge for me before bedtime. I tend to err on the side of too much humidity than too little.
The reason for this is that dry air irritates and inflames sinuses, thereby making them more susceptible to infection and less efficient in filtering pollutants: voila, sinusitis! Also, humid air stalls the proliferation of dust mites (they dig dry air), whose dung creates allergy symptoms, which creates mucus, which can lead to sinusitis. There are ways to check the level of dust mites in your house and to kill them. Here is a web site with the products for that. Pets and stuffed animals are havens for dust mites, as well as bed sheeting, beds and carpet. Here's a good web site for controlling dust mites.
If you get allergies regularly, it's important that you use the Neti pot at least once a day during allergy season, and several times a day if you're outside. Use it several times a day as soon as you feel allergy symptoms coming on. This will reduce your symptoms dramatically and perhaps forestall sinusitis. Also, when you get a cold or the flu, be sure to use the Neti to cleanse your sinuses several times a day to prevent or shorten a sinusitis infection. Be sure to use the Neti, too, after cleaning you home because of irritating chemical pollutants and the temporary increase in dust mite offal in the air.
I've found that staying vigilant in the above has helped me stay healthier and enjoy my life more. Good luck. Respond to this post if you have any questions.