I have used a couple of different methods to get chipped paint look. One that I used recently on a Marder was with salt that I thought turned out very well. It is pretty easy to do. Just put what ever color the base coat on and let dry. Take some regular table salt and where you want the paint chipped put a db of water and place salt in the water. Once the water dries remove any access salt that may chip more than you want. Spray on the final coats and the just take an exacto or something and knock the salt off.
Another way is to take a piece of a scouring pad and dip an edge in the color of the base coat and dab it where you want the paint chipped. That way won't get the texture as the salyt does but still is convinceing. Here are some links to pics of the different methods on stuff I did.
Salt method
http://lizardsworkshop.50megs.com/marder.html
Scouring pad
http://www.geocities.com/lizardqing/panther7.html