Here's a technique I like:
First, paint the model a very dark shade of your base color.
Second, clear coat the model several times to protect the first coat
Third, apply the base coat
Fourth, weather with other airbrushed colors as desired (or omit)
Here's the clincher: Fifth, *lightly* sand through the base and weathered color to get down to the initial coating. Voila...darker panel lines that are easy to distinguish. Too heavy of sanding and one goes through to the plastic, or worse yet, flattens and widens the panel line distorting it.
Hope this is helpful,
Planecrazy