I'm finding that "Bicycle behind you!" (suggested by someone here at the forum) works better than "Passing on the ___". You do also have to give people time to react since they often forget that they're not the only person on the trail , so call out as far behind them as you can and still be heard. Then, let them choose to which side of the trail they want to move.

Though I have had groups of people scatter to both sides and have then had to weave through them... No matter what you call when, nothing's fool-proof.