I once had a St. Bernard mix come after me, I stuck my pump head in his nose and twisted, he promptly went behind his fence and barked like hell, he never came out after that.

Carrying a couple of rocks in your pocket to "bomb" the "bad guys" when they come close helps a whole lot.

A loud and angry rendition of "No! Bad Dog!" will put most dogs "on their heels" to stop, so quickly, it'll make you laugh.