Living in close proximity to a couple of hundred cows, we are often inundated with flies in the kitchen when it's warm outside. There are lots of brands of fly papers about, but we have found these to be the best we have tried. They claim not to contain any chemical insecticide, but just use natural ingredients.
We have tried several other methods of controlling flies, but these definitely result in the highest body count! The only downside is that you have bits of sticky paper with dead flies stuck to them. However, they are cheap enough to put fresh ones up at frequent intervals.
Don't touch the sticky stuff on the paper when you are putting them up. It is very hard to remove it from your fingers!