My friend makes a mix of Listerine, vinegar, and water. In a 64 oz bottle he mixes 1/5 Listerine, 1/5 vinegar, and then the rest is water. It smells awful but it works amazing. He is the only guy out at the stable that doesn't have serious fly problems! Mostly everybody else buys spray at the store pre-made and it doesn't work worth crap, as Joe already said.
Fun fact: Did you know that marigold flowers repel flies? This is why a lot of stables use marigolds to set in pots outside of stable doors. They just don't have a wide radius of repellent. They look pretty though too :)