So in my month or so of doing computer art I've learned some tips that really would have helped me out origionally to use. I hope that you can find them useful too. (of course if they make you say oh duh how stupid could he be then just disregard them and go copulate with yourself vigorously. where were you when i was asking how to do them huh!?)

For smooth lines:
Use the Path tool. you can place the pivots, bend the lines and then move the handles so that you have a really good looking curve in the end. once you have a curve or path ready click stroke path. I like to use the option of 2px width solid color.

To shade within the lines:
Color the layer that you are doing in a base color then use the select by color tool. this will make sure that if you are shading hair or skin you will only put shade on hair or skin respectively. keep in mind what layer you are painting on though.