The following doesn't actually concern the sketch itself just the fun bit, Colouring
A few suggestions.
1) Try to work on layers. Working on layers makes it easier to edit different parts of the image without effecting others. For example you don't have to worry about going over your lines, as the lines themselves would be on a layer of there own.
2)Use the line tool (bezier curves) to define areas rather than just trying to do it by hand. I can see a few areas where the colour doesn't quite reach the line itself. Also draw the lines with the bezier curve to clean it up. Gimp actually has a handy feature if you go to the "Paths Dialogue" window you can actually set a Stroke using paths that you have already created.
3)Clean up. Using the line tool mask round the edges and clean up any parts where the colour has gone over the edge.
http://dsarchy.deviantart.com/
