These techniques work for many materials. If your item is not metal, you can simply apply a layer of metallic paint beforehand. For tips on making plastic look metal, go to the plastic as aged metal page.
Latex Masking Fluid
Apply 2-3 coats of latex masking fluid where you want your chips to be. Apply paint. Remove the masking fluid to reveal the artificial chip in the paint. For a better effect, make your coats of paint thick. Leave your dried masking fluid alone, and instead, add more fluid over your first area, enlarging the area of the chip with each layer of paint. This will result in a quarry-type of effect. Changing the colors of each layer, especially between the primer and top coat, will make it more convincing. Use a pointy tool to pry up and peel your masking layers away. At the end, you can also go over the whole item with a black wash (black paint diluted with water in about a 1 to 4 ratio) to help make the steps between the layers more deep and pronounced.
Other Masking Materials
Other materials you can use to mask the chip areas are rubber cement, petroleum jelly, and mustard.