On the multiuse trails here, it seems there are enough bikers that people understand "on your left." But I've had to give the occasional "LOOK OUT" or "Heads up!" when in a panicked situation..usually requiring repeating the phrase several times until someone gets a clue or I can run off in the grass. Heads up is a phrase stuck in me from a crowded equestrian warmup area. It was really special when an apparently non-English speaking tourist kept crossing the path to take pictures of his wife by the water. Once I observed another biker yell several phrases at him. And then he did the same thing to me!
I often say thank you, in case I sound too mean. But c'mon, I'm a new biker with less than expert handling skills. I'll also appologize if I do something dumb and get in someone's way.