In a regular size spray bottle, I fill it a little less than half way with distilled white vinegar. Then I fill nearly the rest of the bottle with water. Next I squirt a little Ivory dish soap in, along with a dash or two of Avon Skin-So-Soft or citronella oil. Put on the nozzle, shake, and your ready to go!
Like I said before, it seems to work well, and it is so nice not to have to worry if it will cause my horses skin to break out. It may not last as long as the other types of fly-spray, but it seems to last me just fine through an all day ride!