The key is to make it fit right in the first place. Through the process, the skin is shaved thinner, the meat is removed, the skull dries and might shrink a bit, and the tanned skin stretches. All these things make it not fit right. Add to this that not all forms are exact fits to your capes, and that not all of us set the skull plates as accurately as we might like, and you have adjustments to make. So, when mounting, pay attention to setting the plate correctly, and add material to the skull. The material must represent the meat, shrinkage as well as the difference from the now thinned down skin. If, after all this and some good glue, you still get pulling, by all means, add those nails. From there, the theory is, leave them in, as theyll always hold the skin in place. Cut the tops off flush, and then hammer the ends over, the hair will hide them. The key, again, is to make it fit right in the first place, though.