We've already seen a few flies this year. During the summer they can get out of hand, no matter how clean we keep the yard and house. I remember one day last summer where granddaughter killed 20 flies in the house. Of course it didn't help when the cats brought in dead creatures and hid them with us only finding them after they were crawling with maggots.
We've used several different kinds of fly traps, some more successfully than others. I've been to a couple of peoples homes where they had a Ziploc bag filled with water and tacked above the door. Some people put a penny in the bag, others didn't. They all said that this trick didn't keep out all the flies but did keep out most. I'm going to try this trick.
I have several strips of the sticky fly paper. This works well if the flies feel like landing on the strip but more often than not they land close by but not on. The biggest waste of money was the sticky fly catcher that was on a tube similar to using two paper towel tubes taped together. It had little plastic caps on each end. That thing cost $10. It had pictures of flies already on the tubes. I think it caught about 10 flies in 6 months. I can't use these sticky traps in the barn because it's too dusty and the paper doesn't work with dust on it.
One of my favorite traps is the plastic jug filled with the most awful foul smelling attractant. It smells like a dead creature. You have to hang it far away from your house or you will be overpowered by the stench. It works great in the barn but there again the smell is so bad it makes your trip to the barn not pleasant. This year I'm going to have one up at the property line in the back of the property, near the bitter old man's pasture. This is our neighbor who told the grandkids that if the dog came into his pasture while she was chasing rabbits he'd shoot her. (It's an empty two acres: no animals or grass - he disks it several times per year so it's always just dirt, and it's behind his fence and orchard so he rarely sees it.) I told the kids that he is bitter because his dog died when he fed her dog food with plastic that China was using as an additive to dog food. Now he doesn't like other dogs so he just yells at dogs and kids. OK, back to the stinky jug... This stuff attracts flies like crazy. They fly in and can't get out. They end up drowning. One of those jugs can fill pretty fast.
If the jug fills, once they are all dead you can pour out half the jug of dead flies, add some more water, hang it up again and more will come. That's because they are now attracted to the smell of the dead flies.
You can buy this with a plastic jug, just lids that you can use with milk jugs (be careful to not have this in the sun because a milk jug will not last and will break apart), or lids that can be used with canning jars. They also come in a disposable plastic bag. I don't recommend the plastic bag because there have been too many times that the bag breaks.
You can also buy the attractant separately. You can make your own trap by taking a two liter bottle and cutting off the top, then inverting the top into the bottom. This makes the funnel that the flies will fly into but can't figure out how to get back out.
I've also see old fashioned fly catchers made out of wire mesh. They are basically the same style with a cone the flies fly into but once inside they can't figure out how to get back out. I've seen rotten meat placed in these to attract the flies since they don't use the liquid attractant. Once you get flies in and they start dying the smell again attracts more flies.
When I was on the Zaca I came across one man who had his home made fly catcher jar. He used his own excrement for the attractant. Not pleasant but it worked.
We also use fly swatters. I even bought one for the three year old for his third birthday. He liked to play with the one hanging in the kitchen so now he has his own. The older kids will have contests on how many flies they can catch.
I suppose if I could somehow teach everyone to keep the doors closed we wouldn't have flies in the house. This isn't a concept that has caught on. Dead or alive, as long as the flies stay outside I'll be happy.