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I start off by lining up blueprints (or in the case of this car, photographs that didn't quite match) like this: http://darkscarab.com/view/47#step0
Then I create a cube and extrude it out into the rough shape of the car body (really, half of the body and then mirroring it, as cars are usually symmetrical), apply subsurface modifiers to get it smooth, add details from new cubes, cylinders and spheres, and merge it into something printable with boolean modifiers.
You can maybe get a more visual representation of my process by downloading the .blend file from a model like the Hoonicorn and looking through all of the cubes and cylinders http://www.thingiverse.com/thing:967959

Mustang Hoonicorn