Here's a trick for you, provided you are talking about regular flies.
Most answers mentioned a food source. Let's try something else that also attract flies: Light
Flies are attracted to light and will fly toward light sources when their surroundings become dark. Here is what I have had a lot of success with:
Be sure you can create total darkness in the house and only produce light where you want to catch the flies.
At night, clear out or line the freezer with paper or plastic, then, gradually turn on and off lights around one by one to lure the flies closer to the fridge. Then turn off all light source and open the freezer. If your freezer does not have a light, put a flashlight in there, point it inward. In a couple minutes, you will get all the flies in there. Close the freezer for a minute. All the flies will have dropped frozen and not yet dead. Remember, clear out and/or line your freezer. They flies won't get to land anywhere. They'll freeze and drop into your plastic or paper.
You can then collect them, do what you want with them. Feed the frogs, throw them out to nature, or flush them away.
The same light concept can be used without the freezer. You can use a tube, a large, empty soda bottle, a water jug, so long as you make the light lure really tightly controlled so that it's the only light source. Spread sticky glue around the walls & you will get a jug full of flies.