Here is a copy of a word file I wrote up quite some time ago. It is similar to one of the other suggestions.

Panel Lines:
Finish construction and primer coat the plane as you normally would. After priming, layout the panel lines lightly with a soft pencil. Follow the pencil lines by applying typically 1/32" Chart Pak tape. Paint the rest of the color scheme of the model, leaving the Chart Pak tape in place on the plane. After painting, remove the Chart Pak tape. Exposed, of course, will be the primer under the tape. You will have 1/32" wide "craters" where the tape was. With a small air brush, go over the areas that were taped off with a darker color paint than what your paint scheme was, up to and including a very dark gray or even black if you have a dark color scheme. Using a 000 steel wool "sand" the overspray back to the panel line or "tape crater". Leave some darker color outside the tape line, in places, if you would like, for an added weathering effect. Last, add a clear coat. If this is a WWII subject, I add a few drops of black to the clear to further tone down the scheme and adds even more weathering effect.