This question has been asked so many times over the thirteen years I have been on forums and I have had more personal requests for help with it than I can remember.
The main trouble is that in learning to improvise, to spontaneously create sounds deeply fulfilling to your own psyche for a lifetime, you have to do it in a way which runs contrary to all the precepts of Western serial education, how most of us were taught to learn in school. I suppose it also depends on what you mean by improvisation in the first place. At its most satisfying level, it is much more than the employment of aural and mental arithmetic to imitate compositional forms of the past. That is very clever, to be sure, but improvisation at its most transporting is nothing to do with that; it is not a sort of poor man's substitute for composition. Neither are its forms structured like those of classical and jazz. Improvisation is concerned much more with "how" than "what", more with instruction than data, more with dynamic process than a priori static form, more like an organism than a cathedral. It is not hidebound by those sounds produced by music which can be notated, in fact that property, especially in regard to rhythm, sets it apart at the outset.
Do you want to spontaneously produce imitation Chopin, or do you want to express what stillofthenight, and only stillofthenight can conceive, indeed what he (or she? ) is compelled to create ? These two objectives differ fundamentally, it seems to me, in how you might approach them.