Chipped and flaking paint is a difficult technique to master, as it's a strong indication of 'less-is-more'. There are several methods, and I'm sure that wherever you ask, you will get a different answer. As lizardqing allready noted, the salt method is very effective. Base-coat your model the colour you want to show through. WHen fully cured, wet some salt and place randomly onto the model, where the paint will most naturally flake. Airbrush over the salt. WHen dry, you can remove the salt and display the initial colour underneath.
One other, simpler method is to just paint on the chips. You can use a normal pencil (something I haven't tried yet), drybrush with some metallic paint, or paint chips and scratches using a fine detail brush.