People respond better to bells than they do to "On your left!"
"On your left" confuses 80% of the population into veering right into your path. If I think it's one of those 80% that I'm passing I have two options:
1) If they are steadily in the right side of the path, I'll pass without saying anything.
2) If they are weavers or potential weavers, I'll slow WAY down and let them know I'm passing and say "You're good right where you are......" and then "Thanks" as I pass.
Option 3 is the Homer Simpson "OUTTA MY WAY, JERKASS!!" method, but this should be used with caution and prudence.