These are some of the shading techniques that I use with Sharpies. I'll show you how to do the contour shading and a different technique not shown.
If you want a more cartoon look to your drawing, this is how you would shade. All you have to do is draw a few abstract shapes where the "light" isn't hitting and color them in w/ a darker shade.
I don't know if you can see the black highlight within the blue shading, but it's good to combine colors while shading.