No offense meant to Marcos, but scribing the panel lines is a little risky... there's always the chance your scribing tool jumps out of the groove and mars the body surface. Plus, how deep is deep enough?
There's another way to get the same look that doesn't require any scribing at all. I saw a technique very much like this posted by Bob Downie quite a while ago, and it's really clever: Paint the body the usual way, then flow a black wash into the panel lines. Then shoot a final color coat that's been thinned 50-50 with clear, or just color that's been thinned down. That final thinned-down color coat takes away the harshness of the solid black lines and leaves very realistic looking panel lines. If the panel lines are still too black, just shoot another thinned color coat. The only catch is that you need an airbrush to do this technique.