Hey there lordQ,
What I did was lay down a primer/base coat, used small drops of water so the salt adheres to the base coat. I used Pickeling salt, you can use table salt, but table salt dissolves more and can discolor the paint a little. Once the water drys, some salt will fall off right away, don't worry about this, Paint your top coat and once your top coat is dry you can use a stiff brush...I used an old tooth brush, to scrape off the salt, don't worry about scratching your paint job, this just adds to the effect. Some of the salt may not come off, go at it carefully with an X Acto knife and gently scrape off the rest.
Also if you have a multi colored camo, you can put salt on everylevel of paint if you want...so say you have a basic, green, brown and beige camo. Salt on the primer coat, spray your green, salt again, spray the brown, salt, and spray the beige. Once its all dry, and the salt is gone, you will have chips through say the beige all the way down to the primer, but you may have other chips that only go down to the brown or green, same with the other colors.
The only problem I had, was spraying Tamiya acrylics over the salt, caused the paint to discolor some as the moisture in the salt would affect the paint. If you have a very dry area, you may not have this problem. I think my problem was, I used too much water to adhere the salt. My fix was to switch to enamels for that kit, which did the trick, but again with less water to adhere the salt, may have fixed it as well.
If you want to see some pics, check out my Gallery, I did the salt chipping on the BM-21 GRAD track...I think it turned out pretty well, and I would definatly try it again.