A water bottle squirt in the face is often good - it usually freaks the dog out, like "what the h*ll was that?"

If an attack is imminent and you are going slow enough, getting your bike between you and the dog is a good idea, though it can lead to a stalemate. DON'T act scared or turn you back on it, try to be the alpha dog as said above.

I've never tried it, but judging by the number of postal carriers who have a can strapped to their belt, I think pepper spray is probably a good last resort if you are pretty serious about this.