Well if we tell you how to design it, it won't be your design. My advice is forget about it until you see something that inspires you. when that happens jot down the ideas and turn them into a skin.
You can't force it or it will be crap.
Have a look through some skins from some of the great skinners like Heizer, TeazR, schuppor, Maggot on Master skinners. They may give you some ideas.