Well what I do, which seems to be working efficiently, is take maybe five to ten minutes just playing or improvising as quietly as you can. Try to find that spot where you can just get the hammer to kiss the strings ever so slightly. Then practice a little harder than this and then a little harder than than that. What you are aiming for is consistency. You need to try to aim to produce the sound you are aiming for every time. Whether it be ppp or f or whatever. Also, try playing a few simple pieces as softly as you can. It is very difficult! But, as with everything else in piano, if you do this for just ten or twenty minutes each day, your touch will be better in no time. If your used to playing FF pieces then it is hard to play PP. It is much harder to play softer, because you need much more muscle control. For example when working out it is easier to lift the weight fast than slow. This is because more muscle control is needed. You just need to build up these relfexes and muscles and learn how to use gravity to your benefit.