I've never been all that disciplined about music (or anything else, perhaps), but recently I've found my practice habits changing (mostly in connection with working on my "Troubadour Variations", my longest and most sophisticated work for solo guitar, which clocks in at around 24 minutes). I found some validation in the following passage from Robert Jourdain's fascinating book Music, the Brain, and Ecstasy:
Researchers have found marked differences between the practice styles of amateurs and virtuosi. Amateurs tend to play long passages straight through, stopping to repeat faulty notes several times when they encounter them. Virtuosi concentrate on fragments, seldom playing the entire piece, and they correct wrong notes by playing them in the context of a large phrase. They understand that the cause of a bad note often lies not in the motions for that note, but in the motions for the notes around it. And so they correct wrong notes by working on the relations between notes, by reorganizing the deeper levels in the motor and conceptual hierarchies from which the notes arise. In so doing, they deepen these hierarchies and make them more self-aware and manipulable.
I've found myself moving naturally in this direction, which I suppose is a good sign. Jourdain also notes that the best players practice almost every day, at the same time, in the morning. Unfortunately, he says, given the quality of musical instruction, most musicians misconceive their instruments to some extent and therefore experience that their efforts are somewhat misdirected. I'm sure that's true of me, since I'm almost entirely self-taught...