I had a huge issue with horse flies with both my horses and dogs. I started looking at solutions because the horses were running themselves ragged. I learned that repelants don't work because they bite as they land (so no time to repel). I bought a trap, http://www.bitingflies.com/ and it works great. It truly made a dramatic difference in the number of flies in about 10 days. It is expensive, but there is no reason for it not to last. Best part is there are no poisions to hurt you furbabies!
I use Ultra Shield that you can find in the horse section at Tractor supply or any feed store. It can be used on dog's and is water proof. If I have a problem with the fly's I use it every 3 day's. It's worked wonder's for me. I had neighbor's that had horse's. Now they moved so I really don't have that problem anymore
Believe it or not I read somewhere to use Desiten (sp?) diaper rash cream. Last summer poor Rascal's ears got chewed up by flies. I started using the cream and they left him alone! Of course, he looked a little silly with white ears but, hey, it worked.