Your drawings are so great! I'm always amazed by mouse stuff. Once upon a time I could replicate my pencil drawing skill with mouse, but that talent is long gone.
Do you paste in tones for the shading? Like, you have a file of tone and you cut a bit in and paste it over, or do you just shade bit by bit?
This is just MS Paint, so what I've done is to make a bmp image, blow it up 800%, and draw in squares of parallel lines, or equidistant square dots, etc, in several color combinations. I try to copy and paste from those, but often I just have to go in and draw them on the mouse drawing itself. The Janeway is my first mouse drawing with shading.