integra818 asked me how I drew my panel lines so I thought I could share my usual method of drawing them. Not sure if anyone else uses this method.
First you will need a Gundam Marker which is water-based and NOT oil-based. The way to differenciate the two is that the water-based cap has a metal clip while the oil-based has a plastic clip.
The water-based marker's tip is soft (and like a brush) while the oil-based one has a fine metal tip.
First draw a panel line. it doesn't matter if it is too thick cos you can rub the excess off. Do not draw too many lines though. just concentrate on one at a time.
Now rub off the excess using a soft cloth by going in the opposite direction of the line. For example it is a vertical line, you should rub horizontally. If you rub in the same direction, you are essentially rubbing away the ink in the panel lines. Do so quickly after the line is drawn. Wait too long and the ink will be hard to rub off. If you mess up, quickly wet the cloth, clean it off (it's water-based, remember?) and redo.
You should get something like this...
The final result
EDIT: Remember to rub across the panel line repeatedly, until a fine thin black line remains.