the only trouble with dry brushing is that is
really doesn't work on flat surfaces.
edgelighting is a better way to explain things to a beginner: When painting a flat, or slightly curved, surface just add a little white paint to your base colour and put it on the very edge of the surface. If you want the effect to be stronger; add more white to your base colour and put it only the most extreme edges of the surface
Adding a wash at this point is a great way to 'smooth out' the colours: Paint the wash liberally over the whole area and it will give you much more natural shading, even on relatively flat surfaces.