This is a tip I have read on a friends Facebook feed. 
Half fill a zip lock bag with water and add 4 or 5 pennies. Zip it shut and then tie it up where you want to keep flies away.
I have not tried this tip but the horse flies seem to be out with a vengeance this week so worth a try!
There are several theories about why it works including that the light confuses flies with all the light and movement.
