I get chronic sinusitis. I have chronic fatigue syndrome (ME) so my immune system is compromised. At the first sign of a cold, I get a migraine and terrible sinus pressure. However, I have, I believe, finally found something, which works after much research and reading online.
I bought myself a nebuliser for about £40 and some saline solution (I get this in Tesco, boots don't have it). You can also make your own saline solution, there's recipes online. At the first hint of a cold, I use this. I use the full face mask so that I can alternate breathing through my nose and mouth. I do it for at least 10 mins at least once a day, more if possible especially if really suffering, try 3x/day. It keeps the airways a lot freer from mucous. The salt hydrates and cleanses the stressed mucous membranes, making them less inflamed thus opening the airways as well as making the secretions more fluid. I continue to use it to keep myself pretty much cough free and sinuses open. Since using it this season, I haven't had sinusitis or a nasty cough.
If you look online, there has been research undertaken on the effectiveness of saline nebulised mainly for bronchitis with great results and there is some advice for using it with COPD. Nebulising imo is far more effective than nasal irrigation as it also gets into the chest area so deals with the entire respiratory system.
Probiotics ( biokult is pretty good) and prebiotics are very useful. Raw apple cider vinegar with the mother is a prebiotic and helps to kill colds, but I don't find effective on coughs. Take a small amount 3x/day diluted in water at the first sign of a cold. Also useful to take maybe once daily. I eat a lot of raw sauerkraut, even raw carrots are prebiotic btw. Supplementing with vitamin d3 (preferably with k2) is important for good immunity - take with food to improve absorbency. Even if you expose your skin regularly to the sun in the summer months, your d3 levels will have depleted as this takes around 3 months from last effective exposure.
And yes, sadly sugar is evil. Once eaten, i have read, it suppresses the immune system for up to 4 hours.