I put up net curtains (the very fine cotton muslin type which lets loads of light through) on the windows (the whole height of the window) and across the back door on round net rods. The back door one is weighted at the bottom (you can buy curtain weight rope in haberdashery/fabric shops). Very few flies (or maybe only the really clever ones) get past the fabric if they fly in through the open windows.
The back door curtain needs to be weighted or it will blow around in any breeze. If I need free access, I tie the curtain back with a loop of fabric with two curtain rings on, that I hook up to a cuphook in the door frame. (I did have the back door rehung to open outwards so that I can have the door open and the curtain down.)