I've found that many people don't know their left from right, so I opt for a bell and/or "bike passing" followed by "thanks" and/or friendly wave works great. (and if you're one of the wave-haters, you can get bent).

Old video from Japan about the effect of a bell:

Otherwise, feel free to sing:

Take note, neither example uses "left/right"

No mater what you do, you'll likely scare the crap outta a couple people. I've seen people jump and shout at me for every kind of announcement and for the occasional silent pass. However, IMHO, signalling a pass is the best practice for a safe and enjoyable mix use trail. I know not every city is the same, so I'll ask you to announce passing when you come to our Twin Cities. Thanks!