As in all things in life, whether it be art, music, or any particular skill, practice is essential to excellence. Practice is not a destination, there is no end, there is no completion, it is the very vehicle of transformation. There is no point at which one arrives and practice becomes unnecessary.
Ability, in itself, is a seed that must be watered every day, it is a living organism that requires thought and attention. If you were to watch any seed closely, water it consistently, it's progress of growth may not be visible for weeks. Yet, if you tend it, give it ideal conditions in which to grow, one day a tender sprout will erupt from the ground. The time for germination depends on the variety, some sprout quickly, others require more patience and nurturing. The same is true of a practitioner of any art form.
The key to committed practice is discipline. A good musician will tell you, they play every day, not just when they are composing or learning a song. I once listened to a flutist practice scales (going up and down a particular musical key) for several hours. Playing scales is the first thing ever taught to concert musicians, and she was already an accomplished player, yet, every day she would still commit time to this most basic of lessons. A long lineage of this kind of discipline creates fertile ground in which excellence and mastery can grow within an art form.
With visual arts, it is far more subtle. We produce an end result that is tangible, that can be held and experienced. The work and practice that formed it may not be entirely known to someone outside it's field, but anyone with a discerning eye can examine a piece, in depth, and see the skill or artistry that created it.
Whatever latent or active talent we have must be refined with practice. When creativity and inspiration wanes, it is discipline that keeps us working. More often than not, new ideas can manifest during such lulls, but more importantly, the commitment to the overall work should supersede our sometimes short attention span. While this kind of practice may not produce anything worthwhile in the immediate, it's the investment into the body of work that will benefit.
Self examination is also essential. Questioning our work and/or reviewing our progress must be done with detached brutality. In this, it is important to remove our emotional ties, and handle ourselves with a critical authority that isn't about tearing down or deeming, but about growth. Where can improvement be made? While I may view a piece as my finest, never is it without room for improvement.
Often, that critical authority needs to come from outside ourselves. This can be the role of a good instructor. Their illumination of a new technique or process is only one aspect of our learning, we must take those techniques, practice them, and then be ready for sincere evaluation if excellence is what we truly wish to manifest. They should also give us the foundation and example for which we can grow and strengthen our own critical authority. True teachers are simply facilitators of this process. Exaltation should only be emphasized in the work produced, not in the teacher or even in the artist. Why? Because the human ego can crush progress with it's belief of having arrived. If we instead develop a passion for practice and sincere self-examination we will reap a harvest of improvement and innovative distinction.
There is a balance between creative ingenuity and practiced skill. They are like different muscles in our artistic body, and require individual attention as well as harmonious synthesis for health to be achieved. Some artists may be born with a latent ability and talent where this process happens naturally, and with little to no interpersonal influences. In these cases the work speaks for itself. This does not necessarily mean they don't have to work long hours or try harder, it more often means the working and practice comes so fluidly that it may appear effortless.
Creativity and inspiration must be tempered with practice and discipline if we wish to push the boundaries of our own abilities. That is, if we wish to create meaningful art, and not just make objects.