How do you decide where the shadows should go in a drawing, let's say illustration style vs anime? I always have that problem, I have no idea how the shadows, let's say on the face, should look like :O

i dont really know that much about illustration but i do know about lighting because my dad is a lighting cameraman and ive helped him out on a few jobs.
the shadows are always on the opposite side to the strongest light source, it should help to imagine a lightsource, and draw the shadows on with it in mind.
i can explain some lighting techniques but i doubt thats much help.
look at films and TV shows with high production values for ideas and a reference, because they will nearly always do a good job on the lighting.
try X-Files, buffy, sin city.
a lot of "dark" films have good moody lighting where the shadows are obvoius and would be good for reference. nomally though if its not by a director thats either well knwon or as passion, the lighting os just bland filled shdows.
i hope some of this helped, i usually dont post unless i can help in a more definate way.

shadows are difficult part for me....but you kind of have to decide where the 'light' in the drawing hits your subjects and shade in the shadows accordingly, also taking into account facial structure, ripples in clothes, and etc.

what he said. Always plan out your light source before you start coloring, then fill it in in your head and then the basics on paper. I fill and draw out the detail (if any) last. I find it's best to approach shadow in thought-layers from the darkest up that way.. that's myself, anyway and of course.

I hope I was a little helpful. :S

I like drawing for myself, but I am also not good with editing shadows. However, I can give you someadvice, which I have found out.

To put it simple, a shadow is there, where the light can't reach.
Let's say the light comes from the left side.

The left side of the face is lighter than the right side, of course. However, also on the left side are shadows.
Let's start with the left eye. At the side, where the eye is closer to the nose, between the eye brown and the eyeball is a light and small shadow. Also under the eyeball a small one. Don't draw the shadows of the faces too dark! And don't draw the shadows too close to the eyebrown and the eyeball. Make a little space.
In the center of the nose, there is where the shadows of the right side starts. The side where the eyeball is closer to the nose, there you shouldn't make a shadow. Also the cheek of the right side shouldn't be a shadow. The right shadow goes a little bit on the left of the center of the nose, then reach the top of the nose in the center, goes down to the top of the alar cartilage. Don't draw there a shadow! Make a kind of graph to it and draw the shadow to the center of the lips. From there you make also a curve to the chin.
However, watch out. At the edge of the face you shouldn't make a shadow. It doesn't matter from where the light comes from, the edges of the face are always lighted.
The whole upper lip is always darker then the under lip. However, don't forget to draw a small shadow in the under lip and don't forget that there is also a small light sparkle.
To the hairs:
Don't draw each single hair. Make more strand. Else the shadows of the hairs would be difficult. There you can make the shadows so dark as you want, but be careful not to overdo it. The hairs have most of the time oval lighting.

I hope I could help you a little bit. Good luck and success with your drawing!