THis is how i shade, il use GW green for an example.
Paint the entire area in watered down Dark Angels Green, usually watered down about 5:1.
Then mix the DA Green with goblin green about 75:25 and dilute about 3:1 paint this onto the raised areas.
Add more GG so the mix is 50:50 and paint onto the raised areas.
Add more GG 25:75 and add to the raised areas.
Then use pure diluted GG to add the final highlights.
You can then give a very watered down (10:1) glaze of Green ink to blend the colours together.
Hope this helps, think of a pyramid when you highlighting with the dark colour at the bottom and the lightest at the top.
Also if you want the highlights to be brighter you can always add white or black to the colours.
Just remember always allow the layer to dry before painting a new layer. And ALWAYS water down your paints.
*EDIT* This diagram may help:
Blend transition