That depends if your talking recessed or raised.
For raised lines, I will use a regular old pencil-just rub the lead over them and maybe give them a little smear. I will also re-draw them with the pencil where they have been sanded off. I will also use pastels brushed over them.
Recessed lines I usually apply a wash and/or use a in pen right inside of the line.
Two issues ago of FSM also has a couple of excellent articles on weathering.
"I love modeling- it keeps me in the cool, dark, and damp basement where I belong"
Current Projects:
1/48th Hasegawa F-14D- 25%
1/48th Tamiya Spitfire- 25%