Draw real people in real clothing, and draw them well. It's a lot harder than drawing Playboy inflatable women and superawesome he-men, but if you can draw ordinary people well you shouldn't have any problem drawing the ultracliches.
Backgrounds, in proportion.
Draw sequences of three or more whole comics pages, not splash pages or pin-up pages and not one page of this and one page of that. All those tell me is you can fake a page or draw a pin-up, not whether your storytelling and your story sense are any good.
Draw figures so they can be easily recognizable from panel to panel, draw them in the correct proportions, and keep those proportions identical from panel to panel.
Maintain a steady light source in a panel (i.e., make sure your figures/backgrounds aren't shaded from more than one direction at a time unless there's more than one light source in play) and maintain the integrity of that light source throughout the sequence unless the source changes as a story function.
Think dynamically. Really think about camera angle, shading, what will give the shot the most emotional impact for story purposes.
A personal one: draw pretty. Draw work that makes me want to see more of it. I'm dog tired of ugly comics art.
Basically, I want you to go look at the work of the best comics artists in the world. Then draw better than them.
Jag gillar speciellt de två sista, fast jag skulle nog utöka vad tecknare skall titta på till att inkludera även fotografier för att se hur en bra bild är komponerad.