For the actual panel lines themselves, if they are recessed, I apply a wash made from mineral spirits heavily tinted with artists oil paints. They key is to make sure you have a non-compatable gloss finish laid down (like Future) before you start, because a flat finish will just wick the wash where you don't want it. Use a fine-tip brush and touch it to the panel line. Capillary action will draw it through the recessed line.
For preshading areas along panel lines (if you do not have an airbrush), you can use pastel chalks (not the oil pastels). Grind a bit onto a piece of sandpaper and apply the dust with a soft brush along the area you want pre-shaded. Make sure this is done on a flat finish, as it gives the pastels more 'bite'. I use this method often, more so than doing it with an airbrush, because I feel it gives me more control in smaller applications and is more subtle.