Check out this post, 10 Mistakes Illustrators Make, by the award-winning Designer, Illustrator and Art Director at Penguin Random House Giuseppe Castellano.
Some of it's good technical advice (like the issue of getting file types right to the color space problems of how artwork created in RGB will be printed in CMYK.)
Some of it's good artistic advice (a sky is more than just one shade of blue, change your POV, careful where you crop.)
But all of it is really smart to know about.
Read Giuseppe's post, part of his #arttips series, here.
Illustrate and Write On,