The following is an excerpt from an article by Steve Yegge.  While much of the original article centers on the aspects of programming and the underestimated need for programmers to practice their craft, the author makes some excellent observations regarding the way musicians practice. 

How does the average guitarist practice? In the years before I started getting serious about lessons, I played a lot — 6 to 8 hours a day for about 5 years. I learned a lot of songs, all by memorization, and I had to play them constantly to keep them in memory, so at least 2 hours of every day was wasted just running through the pieces. Through brute-force effort I eventually started to sound like I knew what I was doing. Fooled myself and most of the people around me, anyway. […]

The problem was that I had no idea how to practice correctly. The saying “practice makes perfect” is inaccurate, as any music teacher will happily tell you. Perfect practice makes perfect. I’d been practicing sloppily, and had become very good at being sloppy.  […]

Real musicianship is the result of studying and applying the theory, history, and performance of music. Many musicians also advocate studying the physics of sound and music, the construction of musical instruments, the mechanics the human hand and ear, and the psychology of performers and audiences. The average guitarist is no more aware of these sub-disciplines than your average laborador retriever. I sure wasn’t. I just wanted to play guitar. […]

(R)eal musicians don’t practice by playing the piece over and over from beginning to end. They dissect every piece of music into tiny components and work on each one individually — every phrase, every note, every fingering, every transition, it’s all worked through, mechanically and musically, for countless hours. They play it slow, fast, quietly, loudly, even in different time signatures and beats. And they do daily drills: right- and left-hand finger exercises for building stamina and dexterity. Sound like a total pain? Actually, it’s not too bad at all. Takes a bit of getting used to, but once you start doing it right, your overall technique improves rapidly. And you’re no longer “capped” at a particular difficulty level, because you’re not exhausting yourself, and you know how to tackle complex technical passages. Oh, and one hour of that kind of practice is as good as a week of playing songs over and over. […]