I would offer one solid rule, two pieces of advice, and one piece of art for you to judge whether or not to take my advice.
The rule: PRACTICE.
Nothing beats it.
1. A parallel: I play guitar, and have a friend who I'm trying to, not teach necessarily, but guide along the pathway to learning to play. He believes his ability to play guitar rests in equipment. If only he had a better guitar, a better amplifier, a quality wah pedal, better strings, a slide, a whammy bar, etc. etc., he would be good. But he can't even play many chords. It's tempting to hope that there's something outside of yourself, that can be purchased, that will suddenly make you a god of whatever art you love. But all the tools -- pens, paints, lightboxes, Photoshop, etc. -- won't save you if you don't have the talent and know the craft.
Lightboxes are pretty awesome though.
2. Don't learn to draw from comics. Learn to draw from life. In America, we have a generation of artists who can't draw a person, but only a superhero. The mainstream comics artists of the 70s were masters of their craft, but used their talents to create caricatures of humans in the form of muscle bound superheroes. In How to Draw Comics the Marvel Way, the John Romita model of the superhero was a whole head taller than a normal person, and was always drawn at the most extreme point of action. Artists who followed learned to draw from the pages of Marvel Comics, and so their forms were exaggerated, even more so than Romita's. Thus, we got the Image Comics generation, typified by the horrible anatomy of Rob Liefeld.
The key to drawing well is to draw what you see, and not what you think
you should draw. Noses can be drawn in L shapes, yes. But you can't start from there. You have to learn what a nose looks like, what it really looks like, to be able to represent it effectively using a minimalist style (like the ligne-claire of Hergé and his followers). The brilliance of Hergé to me is his attention to detail, then the reduction of that detail to simple, almost geometric lines.
The other key to drawing well is to note the proportions of features to each other. What is the ratio of the eyes to the head, the nose to the eyes, the mouth to the chin, etc. How far from the front of the automobile are the tires? How much space is between windows in a building, etc.? If you have the proportions right, everything else falls in line more easily.
There are, of course, other artists far more accomplished than I on these boards, and I will bow to their knowledge. For what it's worth, here's a recent digital doodle drawn in Illustrator with a Wacom, and colored in Photoshop. I'm not sure that it proves what I'm talking about, but I felt the need to offer something!