Mastering Principles & Elements of Design
Art is subjective. People's taste are fickle and what is in can change in a snap, but what doesn't change is good aesthetic. Design can be thought down to a science, and the study of this science can help improve any artist's overall look to their work. The following are aspects of good aesthetic that I have studied and urge other artist to study as well. But not just to abide by these guidelines forever, but to learn from them so one can use them and go against them like a master.
Create exercises for yourself, practice, & observe them in nature to have them ingrained in your artistic practice.