Design a composition before you even think about adding colour to anything.
Paint your background before your subjects, environment determines what colours are present in your characters, not the other way around.
If using lines - erase scratchy hairs that don't add anything to the drawing and utilize dynamic widths. Messy only works when you still know how to reduce it all to one single line to describe edge. This is a common trap to fall into where some people allow style to excuse lack of knowledge.
If painting - know how to effectively use colour to describe edge.
Know how to pick out colours for shadows and light manually, even if you use tools like burn and dodge. Otherwise these quickly become crutches.
Zoom in and out as you work - looking at it from a thumbnail perspective will help spot misplaced lighting.
Learn how to paint and/ or draw various textures, this is probably the most important. You should not be rendering rocks the same way you render skin. (I promise you will find some really quick methods to imply texture rather than needing to painstakingly detail it once you get the hang of it.)
No matter what you're always going to make some kind of mistake, I think the whole journey is about trying to eliminate the most obvious ones.