As Jeff said, an eraser shield is the ideal tool. Draughting rulers tend to be too stiff. But if you can find a cheap, super-flexible steel ruler, they are ideal for longer lines - like fuselage longerons or wing panels.
Way back when, I used to use Koh-I-Noor 'Rapidograph' pens (with grey ink) for emphasising control surface hinge lines. That worked well ... but I'm betting that some BtS member knows of a more modern, line-line felt pen that would suit.