Experiment with different found objects to find which leave the most interesting impressions. Use the same friskit shapes over and over to create interesting patterns or shadow effects. Experiment with different paints. Watercolors are wonderfully translucent and you can get some interesting color over color effects. Acrylic paints are vibrant and opaque and work on just about any surface.
Enamels are great on metal.
Miller, David and James M. Effler. North Light Books. Owen, Peter and John Sutcliffe. The Complete Airbrush and Photo Retouching Manual. North Light Books, Cincinnati. 1985.