The short answer is practice. :P
The human head is an excellent place to start, you'd be surprised how far you can get with distorting or only using parts of human anatomy. I'd also recommend doing animal studies, concentrating on the overall proportions and small details.
To draw organic shapes, break everything down into simple shapes. That way, if you can render the terminator and reflected light on a sphere and cylinder, you should be able to figure out most complex forms. Imagining contour lines on everything you draw is a huge help too.
For the design part, I think it all comes down to shape design. First make sure the silhouette is a cool shape, then make sure the internal shapes and shadow shapes are nice, then figure out how to make things look good in 3D. Halfway through, ask yourself what animal your drawing reminds you of and use that to tie it all together. Then add some appropriate detail to help sell the idea and make it look a bit nicer. That's how I do it anyway.rezzealaux
Hmm, tough. I think the frustration comes from the gap between what I expect to end up with and what's eventually on the page, and a lack of control over this. I suppose this is just rewording what you said. Often the only way to correct the mistakes is to start over with different proportions, go through the process again, and hope for the best. Reevaluate, rinse, repeat. As this gets better with practice, I suppose I'm frustrated at my own lack of knowledge? And how unintuitive cars are to draw.
The 3 point drawings have proportions that feel subtly off to me, but you're right, they could be iterated on. The parallel line drawings are just easier to do, so they remove the perspective problem while I try to figure out what a car looks like.
And likewise, I don't give a damn about cars, I'm just annoyed that I can't draw them. :P