Tutorials - Shading 1
This first technique uses the Gaussian blur filter. It works best to shade tops (as I've shown here) and other skin-tight clothing.
On a new layer, draw the outline of the top (or other piece of clothing) with the darkest colour.
TIP: If a body part (such as a hand) is partially covering the area of the clothing, draw the outline as if the bodypart wasn't there, and erase when the piece of clothing is done: It will prevent some beige "hallow" of forming around it when you'll use the Gaussian Blur.
Take the third colour and flood-fill it. After you've done that, select the inside of the top with the magic wand tool (in the tool properties tab, set the tolerance to 1). Here, you cannot see the dotted lines around what I've selected because I've hidden them by going in Select>Show Marquee
Take the second colour and draw the shading around the outline. Make this 1px wide in the lighter areas and several pixels wide in areas which should be shaded more (ie: under the breasts)
Add some highlights using the lightest colour. These should go on the opposite of where you put the shading.
Go to Image>Blur>Gaussian Blur and set the radius to about 4 or 5 and the falloff to somewhere around 25%. You can play around with these two parameters but make sure there is no beige around the outlines of the clothing.
After you've gone out of the Gaussian Blur window, press Ctrl+D to deselect the area you've selected at the beginning. Erase anything that covers bodyparts (if applicable) and you're done!
Here's the final doll. See my Shading 2 tutorial to find out how I made the skirt.