Use pastel powder, dry, to simulate smoke and dirt stains etc. (You don't wipe these off, and it's best to do this on a flat coat) - I use a short stiff brush for this.
Use the pastel powder wet (w/ soap) as a wash to get into recessed panel lines. (Here, as Phil_H says, you wipe-off, the idea being, the wash stays in the recessed panel lines.) It's best to do this on a glossy surface (Future). (If you do it on a flat surface, the wash will not wipe off...) - To wipe, I use a Q-tip that is BARELY moist with Windex. If it's to damp, you will wipe the wash out of the panel lines. You can also use a BARELY moist paper towel that is folded and compressed flat, again the idea is you don't want to wipe off the wash in the panel lines.
Both methods take a bit of practice and persistance to get the effect you desire.
Another option is to use a mechanical pencil to draw in the panel lines. Use an eraser if you mess up.
One more tip - If you want to get a FINE panel line. Scribe the line with a slightly dull exacto blade. Then lightly run a pencil across the line. Then rub gently with your finger. The graphite should leave a very fine line behind. Use an eraser if you mess up.
Remember to Future or flat coat after you are done, becuase you might accidentally rub off your work if you handle the model too much.