Moselle, we too have had a serious fly problem this year, and DH finally made some traps that have really worked for us. He takes a 2-liter bottle, cuts off the top 1/3, and in the bottom section he mixes water to the bottom of the "ring" of nubs on the bottle with about 1/4 - 1/3 cup of sugar and 1/4 cup of vinegar. Then he inverts the top and stuffs it down into the bottom section, making a funnel that the flies go down into. The spout of the bottle should be above the liquid, so the flies get trapped in the bottle and drown.
It doesn't smell really nasty like the storebought traps do, and it works really well! The vinegar keeps the honeybees from getting caught in the trap. Just don't leave where animals or people can knock them over!
We aggressively use these in conjunction with fly tapes, and although nothing is 100% I have noticed a big difference since we started doing this. It is very important to get rid of flies, as they harbor a lot of nasty things ~ they are part of the life cycle of many kinds of worms, diseases, infections, etc.
Good luck!
Jen in TN