When applying shadow and highlight, remember to utilize the "five value" system: Darkest shadow, darker shadow, mid-tone, bright, and highlights. For a comic book, you can probably simplify it to 3 values.
I personally get all the shadow taken care of first, then apply the highlights.
The best way to do this is probably post a few images with what you have done so we can all tell you what needs to be improved, if there is any.