I turn and ride straight at them and yell, "NO!" a couple of times. That is almost always enough to either stop them in their tracks or pause them long enough for me to bolt by them.

I say "almost" because I once had a dog that stood his ground and forced me off my bike. He was a big pit bull (I know, so cliché) and I was dancing around, keeping my bike between us. He latched onto my pannier and I was seriously wondering how I was going to get out of this jam. A guy in the next lot came out and I think he clapped two 2x4s together. It sounded like a gun shot and it was enough to make the dog release and run back into his yard. The guy said he had called the police on that dog several times because it attacked his lawn mower (any dog willing to go after a running lawn mower is not to be trifled with).