People were asking for it. So:

Cell shading is the easiest way of shading but still, can be pretty hard. It's a perfect exercise for shading in general, to learn how to fill the form.

Now, some tips:
1. Put around 50% and even more space in shadows. Many artists give shadows about 15-30% only. My own opinion - that's not enough to set up 3D feeling.
2. Don't be shy to put whole limb or half of a torso in a solid shade piece. Many artists makes all body parts equally shaded, what does not look natural.
3. Start with light shadows, then make darker areas. Make as many depth levels as you need.
4. My personal tip: don't make 2 different shadow depth levels with nearly equal areas, it slightly disturbing viewers eyes. Take a look at the picture above:
- First, I put 66% of a character in the light shadows ares
- Second, I used a medium-depth shadows to block out her torso and pelvis topology (about 10%)
- just a tiny little bit of a dark shadows. 3-5%.
So, 66, 10 and 4 - each new level of depth has more then twice less area of a previous one. I can't explain why, but it makes picture look better.
5. Don't use black color for the shadows.
6. All previous tips are not necessary for a background shading.