I have suffered with sinus infections since I was in my 20's so off and on for approx. 40 years.
I recently started wondering if I could be allergic to all dairy, as after eating ice cream I cough and have phlegm for 20 minutes or so. It took a week or so, but since I have not had any dairy the coughing and post nasal drip has completely stopped, and my sinus' have not bothered me at all.
I think if people suffering with sinus infections can give up all dairy (it's not easy), but the reward of alleviating sinus problems and sinus infections is so worth it.