A friend of mine carries a stun gun (or maybe it's a tazer?), and just the noise of it has been enough to deter every dog she's come across. Which is better since pepper spray can come back and get you. (I carry pepper spray currently, although I'd like to get a stun gun instead)
Another thing is that I carry spare slip leads. That way if the dog is relentlessly aggressive and isn't going to stop anytime soon and nobody's around to help. I can tie the dog off to something and do what I need to call animal control. Or you can wrap it around a dog's waist during a fight, drag them to some place to secure them and try and free the other dog.
When I'm walking my grandmother's Toy Poodle I will pick him up. It the other dog is large anyway, as it would only take one serious bite and it could be the end of him.
Dakota - GSD/Lab Cross - CGC CA SRD PD Mondioring OB1 Diabetic Alert Service Dog (Handler/Owner trained)
Alice- APBT mix
Ryuk- Ghost Corn Snake