This method has worked rather well for me. I haven't heard of anyone else doing it like this but I'm sure lots of people have. Essentially it's just a way of manually producing the same kinds of sketches that Sketchup makes, within Photoshop, by using the Transform tool to put a side view of your drawing into place on a perspective grid.
Here's the initial side view drawing. This could be just an accurate technical drawing or whatever. The idea is to provide reference points to draw guidelines through vanishing points.
Side view images distort-transformed into place along perspective guidelines to vanishing points. Perspective sketch built up between images.
Lighting. Shading is laid in on one layer. Fake ambient occlusion is laid in on another layer by putting grey wherever there is a cranny between objects where light can't get in. You can see what the ambient occlusion alone looks like on the top of the vehicle, and the side is in shadow as well. Not all that accurate but doesn't need to be perfect. I got this ambient occlusion idea from Sam Nielsen-- small steps like this seem to be what sell images.
Line art with shading.
Initial texture step. Highlighting and more shadowing. Concrete texture distort-transformed into place.
I hope someone enjoys this. There are all kinds of things wrong with this image but it was just a quickie pitch and at some point you have to quit messing with it and move on with your life.