Music Composers Unite!
As for the emphasis on music theory, since we're talking about classical improvisation, I find it obvious that whenever possible classical harmony should be followed. As always, if something sounds good, but is "wrong" it is more than welcome as far as I am concerned.
Also, I think that a major issue while improvising (in any style), is the fact that you want(and need) to create melodies on the fly.
How can I develop more the few patterns and rythms stuck on my head?
For the record, I was not proposing the adherence to a specific set of rules that dates back to Mozart et al; I am only suggesting the (loose) adherence to some sort of methodology as a foundation for improvisation. In the set of all possible music that you can play on the piano, only a very small subset will sound good. Don't believe me? Get a computer program to play notes randomly on a piano; don't be surprised if it sounds like random notes. ;)
I never specifically said this, but the way I have learned to improvise is a means by which i have developed an intimate understanding of the relationship between different sonorities. Every single musical relationship in music has a specific sound; it's up to the improviser to have an intimate feel for the difference in sonority between (for example) an authentic cadence and a chromatic mediant.
The spirit of my advice is this: take an idea that you apply in your regular composition, and make it secondhand in your playing. Music is at least partially memetic, and a good improviser will usually take advantage of this fact.
This can be applied to all styles; if you want to go 12-tone, then practice inverting and retrograding various series. If you want to play jazz improv, practice playing different scale patterns and some basic licks. When I mentioned basic chord progressions like I-ii-V, they were just examples of how to approach improvisation. Of course one shouldn't strictly adhere to this. That would be silly and quite limiting. I've been very clear on my stance on "constraining" oneself to the rules of classical harmony; I very clearly stated that "my most memorable moments improvising are when I break the rules a little bit." So I don't think we disagree much in that regard.
There are two things that have greatly hindered improvisation by traditionally trained (read: classical) musicians:
1. Creativity is systematically excluded from music education from early on. You play in a band or sing in a choir and perhaps later play in a youth orchestra, but you never receive training or encouragement in exploring music on your own, discovering your own voice. The reason is that creativity is messy. Tricky to teach, hard to grade (less tricky to teach if you don't have to give grades, but we have to grade everything...). It's easier just to learn a system of giving and following orders. A small % play in jazz ensemble, but many just read charts and don't improvise, and the great majority of student players never have this experience. This is also the second part of the problem:
2. Definition. Almost everyone thinks improvisation = jazz, jazz = bebop = 16th notes at 220 BPM, not happening, no way. More exclusion, more hindrance.
To make improvisation easy and natural, we need a new definition. How about this: improvising means you get to choose the note you play.
Use what you know right now. Stay comfortable, gradually expand and explore. Don't worry about harmony for a good while. Start off with creating interesting rhythms (one pitch is enough at first). Then mess around with melody. Timbre. Texture. Improvisation is not about virtuoso playing, it is about virtuoso listening. It's like conversation: listen to what's going on, contribute what you can. You don't have to be an auctioneer. You can (and should) be silent part of the time. You can play stuff that is difficult to write down - because none of it is.
I've had an Improvisation for Classical Musicians at the University of Iowa for almost a decade now and that's how we start. Students (the few brave ones that even dare try) are terrified at first, but then quickly find out that it's both fun and easy. They give three completely improvised public concerts a semester and the results are remarkable. I put together the first five years of the course in a book (Improvisation Games for Classical Musicians) published by GIA. So there is now a guide for those who happen upon it. David Darling has been doing similar things for over two decades with his Music for People; James Oshinsky has set down a lot of it in Return to Child. There is ISIM: International Society for Improvised Music, mostly nonjazz; it has annual conferences where all kinds of folks get together to talk and perform and inspire one another.
One other thing that has been of great use in our class: Soundpainting (see www.soundpainting.com and www.soundpainters.org, also YouTube videos), a gestural system of improvisation in groups, invented over two decades ago by NY composer/conductor/jazz player Walter Thompson (who lives in Europe half the year - the Europeans can't ge enough of SP). Although there are a thousand gestures in the complete language, you can teach anyone a couple dozen gestures in an hour (I've done concerts with 5th and 6th graders after only 2 rehearsals and they were a big hit). The SP web site offers a manual and DVD of 40 basic gestures, which is plenty to give a concert or inject into a class. Composers could make use of SP in their pieces easily enough (Walter has done this) - SP is easily injected into written compositions. SP can also use dancers, actors, and even visual artists, and involve the audience as well. It's a terrific tool for performers, improvisers, composers, educators. There are several SP orchestras in NYC - check them out if you have a chance.
There are very exciting new realms of music and sound possible if 1) performers learn this kind of nonjazz contemporary improvisation and 2) composers can give up controlling every aspect of a composition and make the performer part of the piece. My pianist collaborator and I have done this - we're both composers, both performers. You don't have to write that much - a page or two max, let the performers create the piece anew every time from any of a number of possibilities: melodic motifs, the mood of the piece, chord progression, etc. You can specify something new for the improv part if you like (in my Two Winters I add a two minor chord vamp - 4 bars repeated 4 times per solo). Repercussions was a rondo - ABACADAEA - the ritornello was 3 notes (taken from the opening theme), the episodes were improvised on melodic fragments and mood. Lots of ways to do it. One of my pieces, September Elegy, has more or less entered the standard horn repertoire; it has 4 sections, 3 of which are completely improvised. It's been recorded by me and someone else; the two versions are distinctly different. It's been widely performed - every performance is the same in the mood, but different in the details, even by the same performer. It's been written about in at least one dissertation. The performer is the partner of the composer. This one is easy for classical performers to adopt/adapt to - it's very slow, and the performer gets to choose the notes (including extended techniques).
You could do the same. It means giving up some power and control, but take me word for it that it is exciting to get to hear the different versions - performers come up with stuff that you have never dreamed of. Yes, you take a chance that they might do things that you wouldn't do or don't like. But that's rare. And very worth the great blossoming of creativity that is possible when everybody is a composer.
One reason that classical music seems moribund is not that we need better compositions. We need to make everybody into a composer. It's healthy for people and healthy for society if everyone realizes they have a musical voice, not just a tiny group of elites. Just like it's good that everyone - from childhood on - does athletics or sports. Only a very few become pros at it, but the activity is good for them and they often have a lifelong interest in the sports - which later supports the pros. Just re-creating music in band, etc. is not enough - kids join that because their friends are in it and later quit in droves. If you make them composers and improvisers from early on, they have some ownership of the process and motivation. And if they're all composers, they will look up to and support the pros later on, a life long.