@jsblair
I developed over time a constant drainage down back of throat as a result of 3 sinus surgeries.

After growing up with habitual smoking parents who could care less about their children and second hand some I got constant colds as chid and then into my 20 and later constant sinus infections.

My bronchial problems started with so many sinus infections that would lead to bronchial infections. Now I have tremendous amount of mucus in bronchia area that I cough up. Some times it is very bad at other times not a all.

Some of the things you mentioned I have noticed and been told can cause increase in mucus. Dairy products is one. My PCP says many people are allergic to pine trees, environmental conditions, dusts, and many things that are not tested in allergy testing. So you kind of have to do what you are doing and rule out them by avoiding or not doing things you think maybe causing it.

One poster mention acid reflux causing it. That is correct. My ENT told me there is a silent acid reflux. So you don't even experience the acid and thus don't know it is occuring. I think I have this and one thing to help with this is keeping your head up when you sleep and of course the acid reducers.
Goo luck!!