I'm using Photoshop Elements and a Wacom Intuos 2 to create the images here, but you can use a plain old mouse and any comparable program (such as Photoshop or Paint Shop Pro).
Draw a chunk of hair on your doll, making sure that the edges are wavy. We are doing wavy hair, after all. Make sure it looks nice, smooth and exactly how you want it. That's important if you want your hair to look good. Mine up there is just a rough example, I'd be spending more time on it if it were actually part of a doll. Now, make absolutely sure to lock the layer/preserve tansparency.
Choose a colour slightly darker that your base colour. Using the paintbrush tool, draw lines where the hair sort of folds in. Don't make them too large, but be careful not to make them too small, either.
Choose a colour darker than the last, and draw another line in the middle of the previous with the next smallest brush size. I made this part slightly too large here, you can tell because it doesn't blend as nice as it could. Learn from my mistake. The shadows are now done.
Choose a colour lighter than your base colour, and draw lines though the lighter portions of hair. Don't cover the whole area, but make sure they're a decent size.
Select the airbush tool, and select a small brush size. One pixel should be fine, unless you have a larger doll. Make sure your airbrush is not set at 100% opacity. I think approx. 75% would be alright. Choose a colour that is much lighter than the previous colour, but also not too near white. Run the airbush though the middle of the highlight a couple times. The extra light colour will give it a nice, crisp shine. We are now done. This looks a little too large for doll hair, though, so I will magically shrink it. One, two, and...
Poof! Now it is smaller and should look more like what's on your doll's head.
Using this technique, you can make a doll that looks like this girl. You can make long hair, short hair, pig tails and pony tails. Even just by changing the beginning shape, you can change it's whole "attitude". Wavy hair is very vertasile.
If you have any questions or comments, please leave them on my tag board, and I'll get back to you as soon as I can. *beeeeep* I amuse myself. And of course, please tell me if I'm a good teacher.