Walking stick and pepper spray are great ideas. I carry pepper spray for this very reason. I have used it once and it kept the offending dog at bay long enough for me to get mine out of the area. Have to hit it directly in the face and snout though or it won't have much effect.
If the approaching dog is intent on attacking and you can not keep them seperated, I would give my dog as much leash as I could without letting go and let him defend himself. If you try to break it up, plan on getting bit, maybe by your own dog. He will go into defense mode and will most likely not realize it's you.
The biggest thing you need to do is stay calm and don't panic. I know easier said then done.
Then when it's all said and done, use the walking stick on the other dogs owner for being a retard.