those are good points, jack. while looking outside, look for animal poop. that is one of the greatest places for flys to lay their eggs.
i spray my grass every day, and we have these fly-magnets available at home-depot. they are plastic bags that hang with an opening at the top and some kind of attractant that kinda smells like sewage...
but it works!
also, maybe screen doors would help.
we just put up a couple screened tents on our deck. it has given an area of no-flys for the customers to dinewhile out side.
works like a charm