If you are working from blueprint plans, ensure (before you start modelling) that these plans actually line up correctly. You should be able to drop them into Photoshop and each line should match exactly. When laid out in max, try to insert polys only where necessary to rough out the body shape to begin with, following the contours of the blueprints.
Pay attention to panel outlines when refining and try to keep your edge loops consistent and flowing over the car. Try to keep the mesh as one solid piece and then detach individual panels later on to add refinement and panel depth (as illustrated). Stick to one poly type – quads or tri’s … don’t mix the two types else you will see pinching in the mesh; if you need to mix them, keep them at the panel edges or corners, and really small
Create and apply material with a strong specular material to the object to test out for kinks and dents in the mesh; if you position the viewport accordingly (as illustrated) you should see a nice un-broken highlight as you would on any normal un-damaged car. If for any reason this highlight varies in intensity, you’ve got a problem in the mesh and should be able to immediately spot it and fix it without too much trouble
If you are modelling a car out, start out with the basic shape and slice and refine as necessary. Start out with as few polys as possible to ensure the polys don’t have any kinks in them.