My coach, Tim Crowley's favorite word has to be "consistency". It goes without real elaboration that sticking to a task is the key to success in said particular task. However, I feel like the outliers of the idea of consistency are often ignored - outliers meaning the small day to day consistencies, and then the broader big scope of consistency.
Small, day to day consistency is the drive to keep working every day, little by little, and to absorb the issues that arise, so as to work around them. As a musician, for me this means if practice time gets hit with life issues, I work around it or I work smarter. If I only have an hour or so to work, then I focus on the tough spots and the technical fingerings or what not. The same applies as an athlete; if music demands I spend extra time down at the hall or at a gig, then I alert my coach and we focus on just a few key workouts. This day to day consistency is marked by extreme flexibility and a good sense of perspective. What is important? How can I cope? By prioritizing, I can keep all from just crashing down and losing my consistency.
The next idea is long range consistency. Sticking to a passion when it doesn't feel like a passion any more. Obviously, if you grow to completely hate something then perhaps a reassessment is needed, but if it is just a rut, or even a few ruts, then the key is to keep moving. Often, as a musician, I find myself dreading my pieces, usually before a recital. However tired of them I may be, I have always found that the key is to just keep playing and working through it. Focusing on quality over quantity has helped in the regard too. Quality work and effort will keep the mind focused over these tough times, but also give it a break, to a degree. Again, doing a little is better than nothing at all, and maintaining perspective of the whole picture is a imperative. The proverbial forest through the trees adage. Just keep moving forward, working bit by bit. An inch is still progress, even if you would rather have gone a mile.
I personally attribute any success in my life to maintaining consistency, and thus any failure as the lack of consistency. Last year I suffered some humiliating losses in racing and felt that those were all because I lost consistency with training. Musically too, I find my poor performances come after binge practicing and then the burnout that follows. I see it with others too. Not working on your concerto, then doing six hours a day (the week before). This rarely leads to a convincing or masterful performance.