-- Charlie Parker
According to Merriam-Webster's Dictionary, one of the definitions of IMPROVISE is "To speak or perform without preparation" (http://www.learnersdictionary.com/search/improvise). Most of us think of the idea of improvising as "making something up on the spot" or "spontaneous creativity". How does one practice being spontaneous? It seems like a very contradictory idea.
We know in fact, though, that to improvise successfully one has to prepare. How do we apply this to music and, in particular, jazz improvisation? I like to think of practice in two broad categories -- Technical and Creative. The technical side of practice is where discipline and planning are used. As a trombone player, I dedicate a portion of my practice time to breathing, long tones, lip slurs, etc. I consider this technical practice. I need the basic tools of a decent sound and good chops in order to execute anything on the horn.
The Technical side of improvisation practice means internalizing scales and chords to the point where you don't have to think about them. Practicing them in standard patterns, like up and down, is one way. Another more intense way of practicing scales is to limit your pitch selection in a given scale, and force yourself to improvise within that smaller framework. Be disciplined, and do not allow yourself to play anything outside of the pitches you have pre-defined - no matter where "your spirit decides to take you" in the moment. This forces you to get more familiar with the scale tones, as you do not allow yourself to play certain figures that feel good or lay well on the horn. This is the "practice, practice, practice" part of of the Charlie Parker quote from above.
The Creative side is the complete opposite. Creative practice means that you just play, and allow the disciplined practice to seep in wherever it has the opportunity. This is the "just wail" part of the quote above. It is important that practice is balanced between technical and creative, between disciplined and free. Learn the tools well enough so that you do not need to think about them. A balanced approach to practice allows you to do both.
if you are learning a tune, try playing a solo using nothing but scale degrees 1 and 7 over the entire solo. Scale degrees 1 and 7 may change if the chords change, but be very disciplined about it. Would you ever play a solo like this in performance? Probably not, but you will likely come up with all sorts of different ways to play the root and 7th of the scale that you would not have considered before. This is how we practice being creative -- by forcing ourselves to be creative within very defined parameters, we will ultimately have A LOT more freedom when we "just wail" and do not have to think about the technical stull. Happy practicing!