I'm surprised this one hasn't been mentioned yet: Thin down the exposed edges of panels such as wheel wells, hoods and the body perimeter under the car. You don't need (or want) to thin the entire panel; just the edge, then taper it back to full thickness. Bear in mind that these edges are usually rolled on opening panels such as hoods, or flanged to provide attachment points, as on wheel openings.
Another "reality booster" is underside detail on hoods and trunks, and it's fairly easy to do:
Start by covering the panel with a couple layers of masking tape. Then sketch an appropriate pattern on the tape, peel it off and lay it down on some .005" or .010" styrene sheet and cut around your pattern.
In the example above, I laid .010" sheet over .005" which was cut slightly wider to create the stamped look.