I haven't actually tried this myself, but have seen the results from a good friend of mine who painted a P-47

1) Prime the aircraft with primer, all over, sand as normal

2) apply chart pack tape, tape width defines the size of the line, the location of the tape defines where the lines will be visiable

3) spray primer over the tape areas only. Apply 2 coats if desired

4) remove tape from model, this creates a lower surface that will become the line

5) paint entire model the color you want, the areas where the tape was applied will be have a look of a panel line

You can also do the opposite for access panels, mask the area around the access panel, spray 1 or 2 coats of primer, remove masking. Now you have a raised section that is primer, then the model is painted it has a raised panel.

Good luck!