Have to admit, I think this is the craziest thing, but it appeared to work!
We were sweating at a friends' cookout when we noticed zip lock baggies pinned to the patio posts. When we asked the host about it they said that was what was keeping the flies away. Sure enough when we stepped further into the backyard there were flies all around. So we asked the host the secret.
Apparently, all we have to do is take zip lock bags half filled with water, put 4 pennies in each, zip them shut and pin them to the wall or posts.
Science and Myth Busters say this doesn't work but to our eyes it did...Considering, it doesn't cost a whole lot to do, it might be worth trying your own experiment.