More things I’ve found useful for soft playing.
V. Vertical motion of keys is what creates sound. If you’re moving too fast to play softly, “use up” some of your velocity by changing the finger’s angle of attack. Toboggan down a sloping hill to the key instead of diving vertically.
VI. If a passage is intractable, try it in a different octave to see if the problem’s in the piano and not the pianist.
VII. If you happen to play a note too softly, hold it for an instant after the other notes release, so it will be heard.
VIII. Immediately after striking a long note, reduce the stabilizing force to the point where the key almost starts to come up, and you’ll save energy and reduce tension. This exploits the fact that the force needed to keep a key down is far less than what’s needed to push it down. Practicing this feels nifty.
IX. This video shows the benefit of extending dynamic range downward:
X. The impression of quietness comes either from actually playing softly or from playing with no “bumps” in dynamics. If your soft playing is lumpy and bumpy, but smooths out when you play a bit louder, then play a bit louder, and you’ll better convey a sense of quietness! (Oh, you want to play smoothly and softer? Good!)
XI. Alexander Technique is not a piano technique, but a general approach to using the body effectively. In my first Alexander lesson with Los Angeles teacher Pamela Blanc, I was playing a soft passage from Schubert’s B-flat sonata, with Pamela occasionally just touching my neck or shoulder. After a few bars, I was giggling. She asked why, and I said, “Because I’ve tried for years to create a living, breathing pianissimo—and here it is!”
A mind for practicing: Don’t you love that feeling of being mentally fresh and sharp at the piano? But even when we’re sleepy and slow, we can still find our way to beautiful playing by dreaming as we play. If you think this always results in sleepy, slow playing, you have a pleasant surprise coming! 
