One thing that works very often is a very loud and stern "NO!" at the dog. I don't know why I never tried this until recently, as I have been a dog owner for many years and am certainly accustomed to saying it at home. I have always just tried to get away quickly or am concentrating on not running the thing over, hoping an owner will materialize and call the dog back.

Most dogs have some sort of training, even if it is minimal, and you'd be surprised at how many of them understand and obey "NO!" It has worked for me many times. Like the Dog Whisperer would say, your energy needs to be calm and assertive.

Not practical for most people, but DH carries a very short leather whip with him (he does cowboy re-enacting) and is able to get it out and crack it easily, and believe me, anything with a brain stem recognizes and turns away from the sound of a whip crack, lol.