We always had a big dog in the house when I was growing up and I wrestled with them enough over the years that I'm not afraid of any dog.

As Starx said, they are more likely to go after you if they think you're afraid. Whenever I encounter an aggressive dog, I'm immediately much more aggressive in return: glaring, threatening ("you lookin' at me?"), getting into a fighting position, picking up something to hit the dog with if necessary...

Gary Ray mentioned putting the bike between you and the dog, I've done that and then picked up the bike and waved it threateningly in the dog's direction. Right hand on the seat tube just under the seat, left hand on the bars and held close to my body, and ready to hit the dog's head with the freewheel/rack/rear wheel if it charges.

Based on my experience "fighting" with dogs (more than play, but not a real kill-or-be-killed situation), if a dog really did attack me and I had anything at all to protect my arm a bit, I'd make a fist and try to shove it down the dogs throat. I've seen the gag reflex in a dog make it back off.