Scrawls, sketches, and chicken scratches - a doodle thread

In addition to everything above, it's very important to draw things from life because that way you get a feel for how to accurately draw depth in shapes, rather than drawing from a photo or something similar where the image has already been made 2D for you, if that makes sense.

Also, draw lots and lots and challenge yourself often! I've learned a lot in art school (note: in graphic design with a basic drawing class, so it's not actually targeted at learning to draw better) because the assignments have pushed me out of my comfort zone and had me drawing a bunch of different objects and poses that i wouldn't otherwise. The same thing is happening with my comic project. :>

Basically if you don't know how to draw something (or better yet, don't want to draw something) draw it anyway.