1) carry a frame pump! A good whack to the nose will stop a dog in his tracks.
2) a splash of water from your water bottle. Some suggest using ammonia, but I'd hate to drink from the wrong bottle.
3)Pepper spray - highly effective and teaches the dog to stay away!
4) Pedal Faster!
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger