Do a vanishing point like you would if you were drawing a picture or what mg said. You can also achieve depth with color like tumbling blocks that look 3-D light on top dark to left med to right.... Decide where the sun is in your picture the reflections will give yousome depth. Clear as mud isn't it :? Look up the qulter Katie Pasquini Masapoust (sp) she really adds depth to her work.