In order to attain fluency in improvisation of any sort, whether in an established idiom or in one of your own invention, it seems necessary to begin from a position of freedom and work towards one of order. The trouble is that this approach, with its concomitant rejection of all inhibition and self judgement, runs so contrary to normal musical learning processes, and indeed most other education, that most people find it very difficult. The type of serial, step by step learning we are all subjected to for most of our young lives just doesn't work for an activity whose heart lies in spontaneous and often subconscious response to sound. All the things others have mentioned are good, at least in the sense that they cannot do any harm, particularly eclectic listening - orchestral and world music too, not just piano. There is also nothing wrong with the "bag of tricks" and "mental arithmetic" approaches to improvising; many accomplished professionals are very happy doing that and nothing more. I think that the most satisfying and transporting moments, however, do not arise from that type of playing. Subjectively, the best times are when you enter a feedback loop between sound and action, conscious and unconscious, wherein the impression is that you are a listener, an observer as it were - a mere conduit of an infinite variety of beautiful forms which seem to evolve and multiply on their own.Again, the trouble is that the development of this state as a personal habit requires a high degree of disregard for rules, a complete rejection of inhibition and what amounts to a contempt for the critical faculty. This is exceeding difficult for older people and pianists trained to constantly compare and judge themselves. It sounds naive and trite to put it this way, but what you do is play some sounds and if you like them, then play them again; otherwise play something else. Gradually, over many years, this process results in the establishment of a personal piano language which is linked to the deepest parts of one's psyche. It cannot help but do so.That is how it happened to me over almost five decades, and that is how I see improvisation. Of course I am an outsider artist, so take my remarks with as large a pinch of salt as you deem necessary.