Just some personal experiences;
Ran into 3 black bears in eastern PA, actually my buddy and I were sitting on a ridge cooking pasta, and these guys showed up about 30-50 feet away, sitting on some rocks, looking at us and no doubt thinking that tortellini sounded like a fine dinner. My dog started barking his head of, my buddy started banging on a pot. They didn't exactly run off. but they left.
Olympic national park, early evening, I came across a blackie digging grubs or roots in a meadow. He was maybe 10 feet off the trail that I needed to walk down. I sang "like a virgin" (Don't ask me why that song) at the top of my lungs for a minute or so, and he ran off the trail, although he stood in the alders about 50 yards away watching me, no doubt critiquing my choice of music, which made me pretty nervous.
Last summer (late june) my wife and I were hiking in Glacier. We were in pretty good shape, so we were moving pretty quickly, and about a quarter mile from the car a grizzly cub shot across the trail and hauled it into the brush on the other side. my first thought was "oh how cute" and my second was "oh crap, where's mom?" I pulled out my bear spray and popped the safety, yelling all the while. my wife turned around so we were back to back and started blowing her whistle (I defend that choice by the way, there were hikers behind us with kids who needed to be alerted)we did this for a couple of minutes, until we were sure the local fauna had beat it. The hiking group behind us, who had no bearspray (and whose tween kids were ahead of my wife and I, two snack sized bear treats) caught up to us and we walked to the cars.
Loud noise seems to do the trick, and madonna is apparently aurally toxic to black bears.