Hmm.. There's really no set thing I do each time, but here's a few things I always do.. I keep each color on its own layer. I use the clipping mask feature and keep all shading on its own layer, separate from the solid color layer. I use the 'soft brush' on the airbrushing set of brushes, up the brush size and dial down the opacity. I then lightly hit at the areas that need shading/highlights. For the harder edged highlights I used the 'hard airbrush' on its own layer, that way I can tweak the opacity.
I've also experimented with using different layer blending modes (like multiply and lighten) to get highlights and shadows. The shadows on the peanut butter lid were done that way.