I believe that the key to create your own shaders (among learning from other's) is to understand the physical phenomenons you are adjusting. There is no point on tweaking the sliders if you don't know what they do. Even if you do understand what's going on, you still need to experiment to get the result you want. Just don't try to adjust too many things at a go. And remember that lighting has a huge impact on the final result too...patience is another key word here (as always).