Shallow panel lines

Let the wash dry well, that makes it harder to remove what's in the panel line. Use a damp not soaked paper towel to remove the wash and you can use damp qtips in tight areas. Make sure you use light strokes across the panel lines not with the panel lines, and it may take several passes, just don't scrub. I have no idea what media you are using but this is how I do it with pastel washes.

I agree with everything that Tickmagnet suggested. Usually that is sufficent, but occassionally I've had to deepen the lines by very lightly pulling a scriber along them. One or at most two light passes will do it.

Tom

Some folks enhance panel lines with a sharp pencil.

Also, I think panel lines are often overdone.  On a painted plane, the panel lines only really show that much when the plane is quite old and weathered. If it is a painted finish, and no or only lightly weathered, it may look funny with too noticable panel lines.  Panel lines around removable panels are fine on any aircraft, though.