Intellectually, I think "on the left" is the way to go, but in practice, it seems to cause more problems than it solves... people generally don't process the words, just respond to the sound, which is generally to stop or swerve in your way. People generally aren't on the ball, especially label-*****s in $400 jogging suits blasting Gwen Stefani on their Ipods. I generally just go double wide on the trails.

I'll do "on the left" on a slow pass, where I have time to respond to the passee's response.