I wore a bell for a while. It was spectacularly awesome and ineffective. The best thing I've found is to pick out a notable characteristic of the ped and call it out with a hearty 'STOP!'. I.e. HEY! BLACK SHIRT! STOP! I don't know what it is about the stop command that people respond to, but it easily surpasses YO!, OI!, HEY!, DING!, or any other supposed attention getter in terms of getting them to stand still for safe navigation.