this paint chipping method was featured in FSM a few months ago. First, you spray the model (or the whole area where you will apply chips) with a silver paint. When the paint has fully dried, apply water from a brush onto the area. Now, you are going to need large, flaky salt. Using a damp brush, dip it in some salt. Brush the salt onto the areas where you want your chips. You will have to move them around with a toothpick or another material until they are in the right position.
Once the salt has dried onto the surface, spray the base coat and all the other camoflage colors. Once you are satisfied with your paint job, use a stiff brush to chip the salt off. If the brush doesnt pull it off, you may have to use a toothpick or tool of some kind. The salt acts as a mask, and leaves silver where it used to be. Now you can clear coat and decal your model.
You can use popcorn salt for 1/72nd models. Table salt will also work for smaller dings.