after my basecoat color, i use a wash of either thinned black (dark gray/brown/etc) paint or thinned oils. either way, coat the line and wipe off the excess. the rag cannot reach in the crack so you're left with a nice dark panel line. i've seen people use fine graphic pens to get down in the panel line. i tried it and always seemed to make a mistake and draw outside the lines here and there. i'm sure if i was super-focused and took my time it would have turned out better. i've seen people use pencils and watered-down black chalk dust, too.
experiment on a toy or something and see which yields the best results for your taste.
I want to try a dark basecoat color with light colored panel lines someday like on that neograde valk armor.