A technique that I have used for chipped paint requires some thinking ahead. As always I start with a base coat, then apply the color you want the scratches to be in, usually metallic, or rust color. Then apply rubber cement with a toothpick (these will be the scratches). I like rubber cement because it is hard to work with, and the scratches come out quite randomly. After applying the rubber cement I finish up with applying the final color. This works really well for chipped winter paint schemes!!! Once the final coat or coats are on, remove the rubber cement. Voila, chipped paint!!
A couple notes about this method: Rubber cement is by far the best mask I have found yet because it is to easy to remove. BUT do be careful when applying it, it is easy to get carried away, and before you know it, you have one big paint chip.
Next it is best to do streaks or runs by using a semi dry brush and pulling down from the top. Colors for these can be anything from light yellow, to black. The color would obviously depend on whether you are trying to represent grease stains, dirt that has streaked from rain, rust, or in rare cases running paint. I have also used thinned colors (a little darker mixture than what you would use for washes) for this process too. If you use thinned colors, you will have to use a small amount. You will not be able to just let the paint run down, you need to brush the streaks where you want them.
I would suggest doing your washes and dry brushing after these before mentioned steps. That way if you over do it a little, you can tone it down some.
Hope this advice can be of some use, and good luck.
oh yah, and always test new methods on scrap....