A heavily misconception about shading is that black is darker and white is lighter. While this is right in theory, it looks like crap when applied.

Example:

As you can see, the main color of this ball is red, but you can see how horrible black and white look on it as shading. Black and white, or gray-scale colors will work fine with other gray-scale colors (black, white, grey, etc), but they look horrible with RGB colors.
How can we make it better? Take a minute to think about this then continue reading.

Well it's quite simple, we just use a darker shade of the main color. Take your main color and make it a few shades darker. Don't make it dramatically darker, but make it noticeable enough. Do the same thing, except lighter shades for highlights.

Results:

You can see how it looks a lot smoother and more natural. Of course depending on much contrast you want, you can make it darker or lighter, but we will cover that later on.

Note: This is now the proper way to shade a ball. It is purely an example for using proper RGB shades.

Now that we have some basic understanding of how to use different tones to create shading, lets learn to use them in more creative ways.

Head:

Here you can see I have the makings of a head. I want to add some wrinkles and some brow features around his eyes. You don't need to be a spectacular artist to draw these.
For the wrinkles under the eyes, all you have to do is follow the shape of the eye. I'm going to take a lighter shade of the green and begin to start the highlights of the wrinkles by slowly following the outer shape of the eye. Make sure to use a low opacity brush so the color won't be too hard.

Results:

Now, right now it looks pretty bad, but the shape we need is there. That's what matters, the cleaning up can be done later. Now we need to add the shape. Select a color a few shades darker than the main green (The background color.) and begin shading underneath the highlights.

Results:

Once again, it's not pretty, but that's not the point of this part of the tutorial. You've just learned how to use basic color shading without relying on dodge and burn tools or standard black and white colors. This is a big step in understanding how to shade properly and to develop your own techniques.

I want you to practice playing with different shades of colors to create different are pieces. The more you practice, the better you'll get at it!

In this part of the tutorial, I will be teaching you how to blend colors without the use of smudging or blurring.
How is this possible you ask? The smudge tool is actually a really great tool, but can often take away details and realism. I tend to only use it to clean things up or for special effects. Same thing for the blur tool.

Now, if you don't know what color blending is, it's basically gradients. If you were to take the gradient tool, or blending tool and drag it out across your canvas, this is what you'd get.

Results:

See how clean and smooth this looks? Wouldn't it be cool to be able to do this, except anywhere you want, in any shape or direction using the paint brush? Well guess what, you can and it's fairly simple to do.

Color blending is great for mixing tones together and cleaning up around your art piece.
Now, take a look at that gradient above, there are only two main colors, but because they are blended together, they create several different' shades of the two main colors. How can we achieve this without it being tedious you say? Well it's quite simple.
There are two things you need to know before we start to color blend. Using the right opacity, and how to select colors on the fly, or without having to go over, select your color picker, pick the color, then back to your brush.

To start off, pick a color you like. I will be using a nice deep blue. I'm going to draw the rough shape of a circle on the canvas, then I will select another color and drawing another circle type shape that is slightly overlapping the blue. It should look a bit like this.

Example:

Looks nothing like a gradient does it? Well we haven't started the blending process yet!
Now that we have our two main colors, we can begin to have some fun.
Take your paint brush and lower the opacity to between 20 or 40. I tend to use 20 if I want it to be real subtle, and 40 if I want it to stand out.
For this, I will be using 40.

Next, you need to learn how to pick colors right off of your canvas without switching tools.
Once again, I use gimp, and these tuts are mainly for gimp users, so if you're are using photoshop or another program, this might now work.
Hold down Ctrl to switch to your color picker and then left click on the screen to select the color. Release ctrl and it will automatically go back to your paint brush.

Now take one of your main colors and begin to make short strokes where the two colors meet. I tend to like to make strokes that kinda look like football laces. But it depends on what and where you are trying to blend.

Make sure not to do too much at once, as you don't want to end up extending the red too much. Once you do several strokes, grab the lightest color and continue to do the same thing.

Example:
Note: The black circle marks the lightest color.

Continue to do this until you start to get a blend going. You may want to adjust the size of your brush to get better results when your blending (I'll cover how to adjust your brush size on the fly in another tutorial).
To get a decent blend, it should take you no longer than 5 to 10 minutes, at least for something this size. Times may vary depending on your skill level, and how much you are blending at once.
by the time your done, you shouldn't be able to tell where the two colors were seperated. They should look like that transition into one.

Example:

It's not perfect blending, but you get the idea. The two colors are now one and roughly look like a gradient. Remember, this was all done using just the paint brush. If you want to clean it up a bit, use the smudge brush, or continue blending with your paint brush until you get it the way you want. I suggest the second method. Try not to rely on the smudge brush.
Use different stroke patterns to get different results. Practice this and combine it with the shades technique I showed you above and you'll be able to create some decent art, because both these techniques work hand and hand together.

Make sure to practice this constantly when you working on realistic type art. You'll begin to develop your own style and soon it will become second nature.

Before you even begin shading, the most common thing anyone does is figure how where the light is coming from, how far or close it is, and how intense it is.
Now, you may be thinking "why is this so important?" Well. It can effect the entire depth, feel, and over all style of the piece.

Most commonly in toribash, the light is coming directly from over head. While this isn't mandatory, it is the most practical direction for the light to be coming from.

Example:

Next, if you have a flash light, or some other hand held device that generates light, I want you to grab it and turn it on.
Now this may sound weird, but I want you to point it towards your floor or wall. Move it close to the floor, then pull it really far back. Notice how the closer the light gets to the floor, the darker it gets around it. Well common sense should tell you because the radius of the light is getting smaller. Vise versa when you pull it back, there is more light and less darkness.
How does this apply to a toribash head, or general art? check out the two examples below.

Light that is close:

Light that is far:

You can clearly see the difference between the two. The head with the closer light has more light covering the head, which means less shadows, and the head with the far light has more shadows and smaller area of light.
Using light distances can also be a good trick for creating subtle changes in intensity (as contradiction as that may sound.)

The last thing I want to talk about is figuring out how intense you want your light to be. This is the same as contrast basically. The more intense your theoretical light source is, the darker your shadows and brighter your highlights should be.

Here is an example I did by just playing with the contrast filter in gimp.

Example:

Now, I don't suggest you use this filter to change your contrast, you should do this while you are shading, so you don't end up making parts look black (like in the example.)
If you want more contrast, all you have to do is make something darker or brighter, simple.
Knowing these three things before you begin the shading process will help you come out with an overall better look art piece.

If you're shooting for realism, this is an important step in doing so.
Personally, I don't use this as much as I should, but I understand how it works well enough to teach you.

A lot of people don't understand that light bounces off of objects and reflects on to other surfaces. I don't know the scientific pronunciation for this, but I think you get it.

Example:

Take the picture above for a horribly drawn example. You have a lot of things going on in just this scene.
First off, let's examine our light source and where it's actually hitting.
You can see how the left most wall blocks the light source from hitting the ball. But because it's low enough the light travels over it and hits the right most wall.

Because the light hits the right most wall, it bounces off and hits the ball. If you look closer, you can see some of the color from the wall reflecting onto the ball. The light reflecting off the right most wall not only hits the ball, but the back of the left most wall and part of the floor.
Now, in theory the lighting is correct, but I didn't really do it justice because this is just an example.

Study the diagram ( example) above and figure out how that would apply to your shading.
This probably would apply mostly to metal or smooth like textures or art pieces.

Let's say you were making a robotic/steel head texture. Depending on the texture you are going for, it would either be really shiny or really grundgy and beat up. Regardless there would be some sort of reflection like below.

Example:

It's very subtle, but sometimes subtle things make your art what it is.
Because the light is hitting head on, it bounces several different ways, therefore the gold bounces onto the silver plating creating a slightly glowing effect.
This can obviously be better done, but once again, I am just trying to give you the idea of how to do it.

Like I said earlier, I don't do these things as much as I should, but just because I am lazy, doesn't mean you should be. Take the extra 15 to 20 minutes to make sure all your lighting is practical and you will end up with better lighting.