These five tips are more reminders to myself than anything else.
A) Punch your reader with a strategically-placed potentially iconic moment, that is, 1) a critical part of your story that's 2) not only visually arresting, but also 3) triggers a strong emotional response in your reader that 4) lingers long after your book is put down. It becomes iconic when the reader 5) associates that moment with a real-life event.
B) Make advantages disadvantages, and vice-versa. Add depth to your characters by exploring situations where a character's strength is actually a weakness (because perfection is boring), and where a weakness is actually a strength (because we root for underdogs). And make these developments matter in the course of your story. Strengths and weaknesses can be, but are not limited to, abilities, a personality trait, a status in society, or even a relationship.
C) Apart from dramatic roles, assign thematic roles to characters. Once you've decided on the theme of your story, assign characters who will portray various facets of this theme, with the objective of either supporting the theme, questioning it, opposing it, or offering a new viewpoint. Simply put, a dramatic role is what a character is meant to do in your story. A thematic role is what a character is meant to represent in terms of theme. Not only will thematic roles make your exploration of theme more complete, they will also make your story more well-rounded and thought out.
D) When drawing, make each panel matter. Film directors make each shot matter. Music composers make each note matter. No part of their work is executed "just to get it done and over with." Or, even if they felt that, we ordinary folk hardly noticed. So even if you have to do shortcuts in your art to speed things up, don't make it look like a shortcut. Make it look like part of the grand plan.
E) Draw what you know. If you're not sure how to draw that complex double-vanishing point superhero battle spread, don't even try. Perfect it before printing it. If you want your work to look professional, no matter what your style is, highlight your strengths and hide your weaknesses on the finished page.