I use a bell - several dings - far enough in advance that if they are surprised, they have a chance to recover. Sometimes, when the situation seems appropriate, I will say "passing." Never do I say "On your left" - requires the listener to figure out his/her left, and then what to do once he/she figures it out - "Should I move right (or left) or what??
I never pass without some sort of warning . I had a guy walking straight, and even though I gave warning, his head phones blocked my sound. Without any warning he turned left to be a good citizen and pick up some trash. Fortunately, I had a premonition and was braking anyway, and I missed him.