I've found in my experience that stopping the bike and yelling "Hey!" or "No!" usually is enough. Sometimes dogs are just playful and want to jog with you. Others may be aggressive. I had one aggressive pitbul run out at me from across the street in Louisiana. Luckily, it ran into traffic and almost got hit and honked at. ( Luckily it did not get hit.) But it was enough to scare the dog and send it scurrying back home in defeat.
Here are some videos of my recent trip: (Looking for a world bicycle partner by the way)
http://www.youtube.com/watch?v=PCf-US5moJU