#1 way is to put sand in the coop! I got rid of pine shavings because the humidity in the summer allowed the mosquitoes to breed in them. ACV in the water to prevent breeding. If all else fails, hang a vanilla tree car air freshener in the coop.
I tried vanilla last summer, but I will try the air fresheners to see if that makes a difference. It got to the point I would go into their coop an hour before dusk to spray flying insect killer in there. There is usually so much standing water in ditches around here, sand wouldn't make much difference.