I'm not sure I understand the question - you want to make the lighting more pronounced / exaggerated so that smaller differences in gradient are more noticeable?
With the fixed function pipeline the obvious solution would be to increase the contrast between lit and shadowed areas (which probably means making the light stronger, or your ambient light darker). You could also add specular to your directional lighting to give it a more defined form, but that might be inappropriate for terrain.
Alternatively you could go for something along the lines of a traditional three point
lighting setup. Or you could ditch the fixed function pipeline and write custom shaders to get exact control over your lighting equations.