I find on the 'serious' parts of the bike trail, most people respond well (and correctly) to "On your left". On the casual parts of the trail, and on feeder trails, the bell is 1000x better. Also, I notice the bell gets more attention from the iPod crowd -- it seems to cut through the haze better.

Actually, one of my greatest annoyances on trails are cyclists riding two abreast, so that one is on the center line, thus partially blocking the oncoming 'lane'. I get that they might be coaching a novice, or what-have-you, but really - save that for empty stretches of trail!