What program do you use for colors? One thing i do for a cell shaded look (which I think would compliment your style of drawing REALLY WELL) is to take my flat or base color layer, copy it and set the copy to multiply. then take use a layer mask to make it transparent. You can then paint with black or white on the layer mask to set an area to shadow. repeat the process till you get the desired effect. since the shadow is an alpha channel transparency you can adjust the transparency , color, and saturation for each level of shadows separately.