One method to help create volume with highlights and shadows on models that have large monotone surfaces is to pre-shade the model between the priming and basecoating steps.
For the Fantasy Flight Games Star Wars Legion AAT Battle Tank the primer was done in a dark yellow. Once dry, the pre-shading was completed by airbrushing a mid-tone brown along all the panel lines and an off-white in areas that would catch the light.
The photos below were taken in the airbrush booth (sorry about the messy background).
Once the pre-shading was dry, a very thin coat of Vallejo Iraqi Sand was applied over the entire model.
As can be seen in the photo above, the first coat softened the shadows and highlights, but they still look unnatural. A second thin coat of the same paint was applied.
After the second coat, the difference between shadow and highlight is less pronounced and better blended. Applying a very thin third coat of the same base color makes the blends look very natural.
Some close ups of the basecoat are below. This model is a long way from being finished, but the variable tone basecoat provides an excellent start for the details to follow.