Material Retouch using Material Node with Vertex Color.
Material Retouch is control a shaded area by Material Node with Vertex Color. If you didn't get desired results in a tiny shaded area of a model, do you remake a model? But, remaking it takes time. In other way, Post Effects as typified by Composite Node give a 2D effect, thus, partially incorrect 3D. You can control shading of model as like 3D painting by Material Retouch in short time.
First, paint Vertex Color a shading control area in red and blue at Vertex Paint mode. Paint a bright area in red, paint a shadow area in blue and, paint a ineffective shading control area in black.
Figure.1: Vertex Color painting example.
Figure.2: "Material Retouch" Material Node.
Note: Method of extraction of Red or Blue from Vertex Color using RGBCurve. Extraction of red is zero in all value of green and blue at RGBCurve. Finally, rendering your scene and adjust Vertex color painting and Material Node structure for desired images.
Figure.3: Left:normal shading Suzzane. Right:"Concave Retouch" shading Suzzane.
It's from colors, because Blender 2.45 not supported shader influence controls. Notice, Material Retouch is effect to materials only. Thus, ineffective to Tracing and Buffer Shadows.
Locally Controllable Stylized Shading (English)
Locally Controllable Stylized Shading (Japanese)
Test of Toon shading retouch on 3D (Japanese,XSI)