flies-b-gone.

Believe it or not, it really WORKS! My brother bought a 2-story home and the backyard always have flies. The neighborhood is next to some farms and newly developed land. Naturally, there'd be hordes of flies everywhere! He put ziploc baggies full of water and hanged them on the border of his back porch's ceiling. The result?

No flies.

From the page:

Zip-lock water bags

Many people swear that a zip-lock bag filled half-way with water and attached over entry-ways will repel flies. No one yet knows how or why it works, but there is speculation that it has something to do with the way the moving water refracts light. If you have tried this please use the tips form at the bottom of the page to hare your results with the rest of us.