Here are the painting check list to make you work the best it could possibly be
1) Careless drawing (accurate measurement and such)
2) Too many sharp edges.
3) Trying to paint things instead of shapes (Very KEY*** patches or shapes of value)
4) Painting more values than are necessary
5) Incorrect temperature transition (if you have cool shadow then the shadow stay only cool shade)
6) Muddy color or not confident blurry color
7) Unsuitable brushes
8 ) Working too much on detail instead of focusing on the overall image.
9) Overwork what should be left alone.
10) Values and edges
11) Too many highlights or hot spots.
12) Aimless brush strokes hoping to get lucky.
13) Poor selection of pigments and palette.
14) Timidity and lack of confident (fear of making mistake usually resulting in muddy)
15) Color should be clean and clear