i also use a similar method. an enhancement to the method of sketching you are using is to not use black and white, but to use colors closer to the final colors you hope for. this image is great, but consider the light sources, and how the elements of the image merge together. if the moon is not shining on the character or the environment, then only half the possible effect is achieved. In this composition, with a large moon like this, i would expect a sihouette with a glowing outline. This is actually lit well for a moon, but not from behind. Also keep in mind the effect the night and the available light has on the colors in your image.