Usually having more than just a couple flies in the house means that you have something they want to eat. If they are small you probably have lost a banana or potato behind the cabinet. If they are larger houseflies, you may want to take out your garbage and look for things that are rotting. And if they are blowflies, you should remove the corpse from the guest bedroom. Eliminate their source of food and clean up any dishes or trash that could become a source of food and they will go away. (If you have little clouds of them you may want to use a bit of bug spray, too!) If you have just one or two flies, they probably just came in with the door opening or through a hole in a screen or something, try getting a butterfly net to catch them, since it's a whole lot easier to catch them from the air than it is to swat them when the land briefly! Good luck!
It only takes one fly to slip into your house from outside and then start reproducing. They eat garbage and food so they just keep multiplying if you do nothing. The clear sticky fly strips you put in the doorway have worked the best for me short of bug bombing a house.
I know in my opinion because I do thus a lot it is DRUM ROLL PLEASE leveling the doors open and things so the flies can get in!
I came home today and found 8 flys on the curtain in my bedroom really freaked me out.. My windows were closed
Food laying around... Ummm leaveing a door open