Hi.
Playing softly is - to most people's surprise - one of the most difficult aspects of piano playing. Since soft playing sounds like - if played softly in a right manner - some kind of tenderous touch which doesn't involves much effort. For example: listen to the second movement of Mozart's Piano Sonata, K.280 (the F major one) as played by Glenn Gould. It sounds like as if the notes are played complete effortless and untensioned.
Now that last thing is very true, but the first one is far from the truth. In fact it takes a lot of effort to play softly, but it takes a very different kind of effort then when playing ascending 32d notes octaves (as you will find in, for instance, Liszt's music. However. This is how I usually can manage to play a certain passage softly in a correct way:
1. Before you even touch the piano, hear in your mind the music you want to play. Hear it exactly the way you want it to sound. Then after you've done that, you can play the music. Be sure , when playing the music, you always are one step forth in your mind. So be sure to hear every bar or passage in your mind first, then play it. This also mean you have to hear other music in your mind then the music you're playing, which is a mentally hard task. You have to practice with it first.
2. Be sure that you're fingers are very active. This is achieved by curving the fingers so, that you have full controll over it. Believe me. You will be able to play softly much better when curving you're fingers.
I hope this helps.
BW, Marijn