Sometimes it's nice to just work in a very pure way with maybe one medium, in this instance I am referring to ink, permanent ink and an old fashioned mapping pen.
Here's what a mapping pen looks like, except I use a wooden handle:
It took me a long time to master it and to find the right sort of paper to work on - anything with too much of a tooth and the nib gets stuck and sprays ink everywhere! The beauty of mastering it though, means you get a wonderful flexible nib to work with and can make some very interesting and fluid lines.
Here's an example of an illustration I did for Hodder children's books that is rendered in this way: