Dr. Lenher from Deer Creek Equine says this is good stuff. Credit goes to Dr. Sally Banner though.
HOME MADE FLY SPRAY
2 cups white vinegar
1 cup Avon Skin So Soft
1-2 cups Tap Water
1 T Flick's Fly Spray
To make a gallon, multiply all of the above by 3. Put in a spray bottle, and spray the horse.
Wipe down horses who don't like being sprayed