This is a very interesting question with numerous answers I think.. Firstly, what constitutes development as an improviser? I think this is almost entirely an individual subject, because what is "improvement" for one might not be "improvement" for some other. I guess it sort of relates to what stage you are in in your musical understanding. A beginner would do well to look at the basic major and minor chords and the accompanying basic scales and play as much as possible and look at some technical studies, someone more advanced within their musical journey; might do well to simply listen, play, be exposed to all kinds of music, in other words strive to "live" on the edge of your comfort zone so as to expand your musical boundaries. So development is the road you are on when your improvisations start to sound different from each other every time you sit at the piano. I like to listen to music from all kinds of different eras, not only classical but also jazz and some experimental electronic music, which can give you new ideas and insights into rythm, think Aphex Twin or Amon Tobin, for instance..Twin has a nice new(well it's not so new anymore) album called Druqs, where he in many tracks simply plays his own modern piano pieces, some of them were revelations in their simplicity to me!
For myself I've noted that whenever I try to improvise something after having looked at, for example a chord book, my impro's tend to get bound and restricted by that knowledge, and I have less freedom to flow at my instrument. The greatest improvisers I've heard(and seen live) have never been big on sitting with their noses glued to theory books, mind you. They have learnt some basics yes, but after that found their own ways. Lastly I'll say that any development never comes overnight, it may take years to go from one "level" to the next, the patience of a (-insert very patient animal here-) is required.
Personally, I've come up with my best stuff when I have simply trusted my musical subconscious and taken the attitude of, "no mistakes, simply different paths".