I wrote a few weeks ago about The Six Pillars of Musical Development. I ranked the most important activities for mastering a musical style, with jazz as my personal focus: 1. Experience (see live music), 2. Listen (seek out great recordings), 3. Interact (perform, jam), 4. Imitate (transcribe, cop licks), 5. Create (compose, improvise), 6. Practice (theory, books, everything else).
In conjunction with identifying these broad elements of pursuing musical mastery, I also broke down the most important aspects of practicing. This includes the final three "pillars" above: Imitate, Create, and Practice. The first three are straightahead; just engage yourself with the music and its musicians. The latter three, under the larger umbrella of "practicing music" require more planning.
I'm discovering that this is the most essential practice activity in my arsenal, and the process is more involved than simply recording the notes of someone's solo. Check out this article by Dave Liebman: The Complete Transcription Process. I haven't been through Liebman's entire process (maybe soon), but the point is to spend some extra time dissecting and internalizing the solo, getting into the player's head and fingers. As Liebman suggests, it's cool to have a few transcriptions at various stages of the process. I've been slowly plugging through Pat Martino on "Just Friends" and John McLaughlin on "Very Early" when I can make the time around schoolwork.
Including sightreading in one's practice routine is a personal preference. Many genres, particularly those employing guitar, don't require this at all. But it also depends on the player, his/her approach to music, and the level of mastery he/she seeks. I include this partly because I'm tested on it at the beginning and end of every semester. I also just want to be a better reader, both for the sake of enjoying the process of learning new material and for future career opportunities that require it.
There are as many different approaches to composition as there are composers, but a universal tactic is to do it as much as possible. Writing something new every day is probably the best way to increase compositional confidence. Not to be overlooked is the necessity of reviewing past-written material, whether to gain more ideas, refine weak areas, or simply to learn your own tunes in order to perform them.
While I'm still in school, I won't be making a living performing music yet, but it seems like the best asset one can have is a strong repertoire. Whether it's classic rock, blues, jazz standards, or campfire songs, carrying a huge catalog of music in your head is a boon to anyone who hires you, often more so than killer chops, sightreading skills, or a unique sound. See my personal approach to building jazz repertoire in a couple of my past posts: Learning Standards Again and Counting Tunes at Jam Sessions.
It's easy to get sucked in to buying a bunch of music books. I do it all the time, and I have a massive queue of texts on my desk shelf. My trick is to focus on one at a time without getting distracted by the others. Must be a poor trick, because I always get distracted by the others. Books are probably overrated, but I don't like to discount them completely. They represent a great static resource that may present you with new ways of thinking about musical concepts that you wouldn't pick up from listening. I just finished working with Liebman's A Chromatic Approach to Jazz Harmony and Melody for school. That one's pretty advanced, so I'm sure I'll return to it someday when I get a better handle on tonal playing. Next up is a nice short one from Tom Dempsey, The Jazz Guitar Experience.
This miscellaneous category overlaps with the others, and can include anything you like: scales, arpeggios, improvisation exercises, new techniques, etc. I currently include most of the material from my private lessons with Bob Magnusson, Rick Helzer, and Richard Thompson in this category. I do a lot of little improvisation exercises, ii-Vs around the cycle of fourths, upper structure triads, slow chord scales over standards.
Now the hard part: working on each of these areas every day. I came up with this plan almost two months ago, and I've only had a handful of days in which I strictly addressed all these elements. I made a daily schedule of 4.5 hours, consisting of half an hour of sightreading, half an hour of repertoire, an hour of transcribing, an hour of concepts, an hour of composing, and half an hour of books. I've still been practicing regularly and targeting this master plan, but I want to get myself used to a routine that includes all these ingredients.