start by making some edges where you want the cutlines to be at (split poly tool, or cut faces tool, depending on your need).
then select all the edges that make up your cutline, and bevel them a small amount, to be as thick as you want the separation to be. Maya's default bevel tool isn't very good, but theres a good one HERE for free.
now select the new thin row of faces, and extrude them inward. then with them still selected extrude again just a tiny amount. this will help keep the edge sharp when you smooth the model.
now select the edges on both sides of the recessed line (that are level with the body panels), and do a small bevel on those, once again so that smoothing will keep the edge sharp.
Here's a rough idea of what you should get when you're done. (click thumbnails for big pic).
unsmoothed.

smoothed.


hope that gives you an idea of how its done
if you want the edges even sharper, do some more small bevels on the edges.