Man, I wish I had some pics of a few cars that I have incorporated this technique on, but the way I do it is to take some salt (not too much) and a few drops of water to make it into a slurry type paste. I use an old or cheap paint brush (you will probably ruin it anyway) and dab it into this substance. When there is a decent amount on the brush, you apply it of the car. I "dot" it in certain areas I want the original paint to be left intact. Afterwards, I take a hairdryer and lightly use it to dry out the salt. After the salt crystalizes on the surface I will paint the car. The salt will not be affected if you use an airbrush on it properly. After the paint dries, you can rub the salt off, and it will infact leave jagged edges that are VERY similar to chipped paint. Try it out on a piece of scrap styrene or an old car to master the technique...Its real easy and can be added with other simple techniques to add some various weathered cars to your fleet. Hope this helps! Also, as stated by Smoke, modeltrainsweathered.com is a GREAT website for learning new techniques just like that...
-Rich