While I suspect that 1Up has his own answer, I can say for myself that taking away the pixelliness of a piece has all to do with 2 things : learning by eye, and never banding. Banding and jaggies (two parts of the same problem - banding is just jaggies over multiple colors) are the single largest giveaways to pixel art. How work work shadows around and over outlines and how to bleed over highlights to create the idea of an emergent (not delineated) form is something you learn with time and practice.