I read about an excellent ballasting method in a special issue of Model Railroader. Basically, after positioning the ballast exactly how you want it, you then soak it thoroughly with rubbing alcohol. Then, you cement it with a 50/50 mix of white glue and water. It works extremely well. For some reason, the rubbing alcohol holds everything nicely in place. Then the glue/water mix soaks in and holds it very well in place. I can provide more info if needed, or look up the special MR issue -- it came out about a year ago. Cheers, Rob