Get serious and catch flies by the ton.  We have 12 chickens so flies can be a problem.  3 of these gallon traps really do the job.  The only issue to remember is that if you hang the trap in direct sun, it may overheat the bait, which shortens its attractiveness duration.  And sometimes the bait just doesn't seem to be as potent as other times.  But when it works, the flies come from great distances and get caught, so it is worth it,