Having suffered the ravages of flies,I came across this on another forum and 'borrowed it'

water for fly control:
Thank you for your wonderful site! I found it very informative. I wanted to let you know about a fly repellent that is is very effective. A plastic bag filled with water and hung will repel flies from the area. I learned this trick while living in the Ozark mountains where the fly problem was horrible. I saw these bags over every front door and on every patio. Supposedly the fly can see his reflection in the bag and does not like it. I am not sure how true that part of it is, but for whatever reason, the bags really work. Thanks again,
Michelle