It all comes with practice, just don't be afraid to try something new or even to drastically change your work mid process or even after it's done. And don't critique yourself too much while working as it can stop the workflow, but once you're done just analyze the hell out of your work and be as harsh as you can. Judge the light, how constant it is and how does it relate to it's sources, the ambient, depiction of scale, anatomy and such and then start fixing it (especially if you're working digitally).I cannot tell you how much time i spend (after i finish something) just working with the hue/saturation on different layers in photoshop and even setting many see-through layers with barely visible adjustments to the ambient and such.But basically the best advice I got, that worked for me is ''Don't just work with your feelings when you approach your art, but THINK about it!''Rant over