funny, I looked into this a couple of months ago by doing what mop suggested. From the side profile, current cars appear to have the A pillar (windscreen) pointing to the front axle, the B pillar (central column in a four-seater) pretty much inline with and on roughly the same angle as the driver's seat. The C pillar is usually where the design styles differ. i.e. Sedan / SUV = passenger door terminates over the rear axle in family models. (here are a few shots of mine
for example.) The tail of the C pillar (and rear window) tends to terminate at roughly the same place the rubber does. Sports cars have varying mergings of B+C pillar which are usually delineated by follow-through from the apex of the roof line (driver's head) and predominantly has the leading edge pointing to around the rear axle, and trailing edge well above the rear axle - this is what makes a sports car look sleek or agressive i.e. Lamborghini countach / gallardo.. Ferrari 360, Lotus Elise / Exige. Small Hatch = Door cut points to rear axle, C pillar commonly starts over rear axle and lends support for the rear door. As Erol said check the Blueprints at www.suurland.com
for the mercedes A class, C class, and Mclaren F1 (the leMans car - not the Formula-1 ..) they're examples of what I'm talking about. Of course the further away you move from the norm is where the signature style comes into play for each brand.
If you're trying to do something like Harald Belker's red minority report car, note that the driver's seat is so far forward that the vehicle lacks front mudguards to accomodate driver access which would not make it practical for public consumption. The newer egg-shaped designs kicked off by Belker's influence on Porsche in the '90s look like attempts to push out and round off the A and C pillars at their respective wheel and integrate with the bumpers. Start from there if you're aiming for the futuristic look.
The "Sketches and rendering" and Techniques -> "2D" subfora at www.productdesignforums.com
seem to attract artists that are specialising in industrial design and implementation. Stuff like this
.. and this
and at www.cardesignnews.com
under the portfolio tab you'll find some futuristic automotive student designs.
I'd start with making a wheels + seats template from some of the suurland.com pics and try sketching your own overlays.