I used to not use a bell, figuring that my cranking along through the rocks, roots, etc. created a noise akin to a threshing machine with a lot of loose parts would be enough to warn everyone within half a mile—-then the years passed, with my hearing gradually going the way of all those older hikers/strollers–so I, too, now have a bell and give a quick ring when approaching.
The old standby “bike on your left” or “bike on your right” can prove problematic as I am pretty sure some of those people either don’t know their left from their right—or they misinterpret and move left when you say “on your left”