Right?!
It doesn't really matter what you say. People will react in the exact wrong way. 
I've been lucky enough so far with "On your left", but I'm dreading the first time that someone walking their dog or someone with a kid on a bike veers left when I say that...
Generally, I think it's good form to inform someone that you're passing them. It doesn't assure you that they'll react well, but at least you're doing what you can. If you get the sense they know you're there, you can pass relatively close and give others coming the other way the room to manuever. Otherwise, pass as far to the left as you can safely pass.
Tom
(Cat person who really doesn't much like dogs....)