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.