When you hear/see them coming, stop and get off the bike if you have time. Point directly at the eyes of the lead dog and look straight into its eyes and yell "NO" in a commanding voice. I've made the toughest looking dogs back-pedal like Scooby Doo using this technique. One big farm dog in Nebraska actually left blood spots on the pavement from sliding and trying to get turned around. So far, it's never failed. I have a small container of pepper spray velcro-ed to the seatpost on long trips, but I've never had to use it.