Soft Backing Music
Another great tool to train yourself to play softer is that when playing to a jam track or a study song recording, simply set the volume at a low level; this forces you to play softer in order to track along with the recording. I also like to have students plug in and play amplified, but set the volume of the jam track (or me on the bass) at an acoustic volume, again, forcing them to play softly. Too often playing amplified is only serving the purpose of being louder, but it's so much more than just that... the change in tone being of most importance. This is great practice for both learning to play softer over all, as well as playing dynamically.