I'm from the "I'm going to be passing on your left side" school. Even that does not have the expected result sometimes. People (including slower bikers) often startle at the first sound because they were totally unaware that I was there. In this case, it's still a nice phrase because they quickly realize that I was trying to be polite and informative. Many however are wearing headphones and will not hear any warning at all. For those I just slow to minimum passing speed an hope that they won't jump the wrong way.

I have a mirror even on my dirt bike so that I can see behind on the trail. Even so I have been startled once or twice by bicycles going by at a high rate of speed that I was unaware of. It happens, so I understand the plight of those I'm passing.