I'm not a comicbook artist. But you could try drawing your lightsource on the paper itself, after you've drawn in your figure. (draw a lightbulb or just a dot where the light will radiate from) now figure out where this light lies in space?(infront of the character? infront to the side? behind? above and behind? above and infront?) Now whatever you've decided to go with, realize the figure is made up of very basic forms, Cylinders, Cubes, and Spheres. (arms can be cylinders, head can be cylinder, sphere, cube, whatever) Now its just a matter of understanding how light and shadow work and applying it to those basic forms. (i would say work from the largest masses to the smaller masses)
here are some examples, notice how simple and how basic he keeps it.
the cones floating in space are the indications of his light source.
also a sexy quote i just saw yesterday.
“ It is very good to copy what one sees; it is much better to draw what you can't see any more but is in your memory. It is a transformation in which imagination and memory work together. You only reproduce what struck you, that is to say the necessary. „ -Edgar Degas
It helps me sometimes to do quick sketches and apply very basic shading and shadows to them. Map in lightly where you want the shadows to be and then just fill it all in really quick. Also studying different types of lighting situations might help out.
this is where studying anatomy and Knowing the major forms of the body are Important, not just in the figure but lighting as well. visualize your painting as simple as possible then just simply block in shadows, to know exactly where to put them just requires lots of practice and patients studying objects and figures. I'm sure you have a sketch book. There is know substitution for learing from life. when your sketching pay close attention to what the light/shadow is doing study those forms.
Keep using reference. The more you use it, the less you will need it when drawing from your imagination. After I'm done a line drawing, I'll grab some reference before I start to render it out. It makes my work so much stronger.
Also check out this sample from Pixel Corps. Iain McCaig did a lecture that they recorded. It's long, about 20 minutes if I remember and you need quicktime but it's drawing 101 by Iain McCaig. Near the end he talks about shading. His analogy is classic. It's like you're in the shower and the water falling down on you is light. Iain explains it so much better than I can. http://www.pixelcorps.com/info.php
Learn to conceptualise the planes of the head and body when you draw from life. When working from life or reference, the light gives you the planes. When working from your head, the planes give you the light.