People always get the misconception (at least in my experience) that when we say "On your left", all they really hear apparently is the "left" part, and proceed to move left, directly in the path of where I'm heading. I have found that saying "passing on your left" works better. Of course though, there is always the swift and stealthy method of not saying anything and blowing right past them with ample space. I have had times, though, where passing without saying anything results in them yelling at me or me hearing them gasp/surprised as they're behind me.