Couple of different methods I have used are:
Run a very fine pencil, preferably quite soft, down each panel line, then make sure you varnish imediately to prevent smudging.
Apply a gloss varnish coat to the model, then with a cheap water colour set mix up a thick paste and apply along the panel lines. Allow this to dry thoroughly, then with a piece of damp kitchen towel rub the paint off, making sure you rub at 90 degrees to the panel line. This should then leave paint in the panel lines. Once complete simply apply the top coat of varnish.
Alternatively use a very thin mix of dark acrylic and water, and simply apply to the start of each panel line. Capillary action then draws the mix down the line, and dab of any excess with a cotton bud. Find that this works really well on wheel bays and gear doors.
Karl