When it comes to lining art ink can be your best friend. 

With it's sharp ends and how it won't fade makes your art pop out to the eyes. 

You can use tracing paper and line on that following your sketch, or even just rub out the pencil to fade it before using ink.

There is actually a month dedicated to ink, and it goes by the name of Inktober. If you can't guess by the name this goes on during October.

Of course there is many different pens you can use, some are used for Calligraphy, others you use everyday at work or in school. You can get special ones in the art stores with different sized pen tips.

It is best you take care of these and keep the lid on them when you are nto using them, so they do not dry out.