You might want to fix your lights and front grill. They are under the color so they'll be green on the car. Just move those layers above the color layers in the Layers window.
Mostly its because of the resolution the skin was created at. That last skin I made was at 2048x2048. Also, using vector graphics as much as possible for logos and decals helps very much (if you can find them).
You don't need to 3D Render to get a good preview.
Crank AA and AF up to max in your graphics card settings, turn the text/menus off in the viewer using the space bar, and fiddle with the lighting a bit...
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The guy that made the Tokyo drift S15 skin... It was nice like the other guy said. But the dude that made that skin below, it needs more practice...