i had a nasty fly problem last summer. i found what worked, place a banana in a bowl and put this in a tight ziplock bag. take a fork and poke holes in it. the flies will go in but not be able to get out. put the bowl in the freezer at the end of the day to kill all flies before discarding and start again. place several traps around.
like cindy said, you must clean everything. they can even be breeding in your kichen sink drain. this was my case, they were comming out of the drain (took me awhile to realise this though...). while you are setting traps, you could try plugging up the sink and keeping a small pool of clean water in, just to prevent any flies from getting in or out. cause they just kept comming back in swarms until i figured out they would go in there. i never knew this before last summer, but supposedly the sink drain is a common source for these pests. i used boiling water to flush it out.