The thing with mace or pepper spray is that you could inadvertantly spray your own dog, or the wind could even blow it in your eyes.

One thing I've heard of is a citronella spray called direct stop. It comes in a small canister, like pepper spray but it's the same stuff that is in the citronella collars. Most dogs don't like it.

Direct Stop

I also know some people that carry a big stick with them. I like to carry my leather leash with me, doubled up a few times. I hit a dog with it once when it charged my dogs and it worked.