>>10862
First, focus less on your lines and more on your 3D shapes. If there's something, e.g. anatomy, you want to draw attention to, putting it right on the edge of your shadow is one way to do it.
Second, think about your light sources before you start marking light and shadow. It's easy to go by feeling alone but that doesn't always work out right.
When you have a spotlight and a direction, you can think about how the surfaces you're trying to light are angled. This tells you how broad you can make your bands of light and shadow, as well as how sudden the change in lightness is when you move along some direction. Remember that light has to be
reflected into your eye so you can see it.
If you want to get even deeper into it, don't just think about spotlights, but also the diffuse, ambient lighting that exists in most of your scene. It's also influenced by the colored surfaces that surround your object.
Lastly, think about your palette. What colors you pick for your highlights, midtones and shadows is defined by the material you're shading, as well as the colors of your lights. (Ambient lighting can play quite a big role here.) Usually, you'll want your shadows a bit more desaturated. This is basically a question of experience, but you can pick up a lot of it simply by looking at things.
I played around with your shadows a bit, have a look.