A wall, a nail, a hanger, and a hammer: Hanging your favorite piece of art on the wall seems so simple that even Paris Hilton could do it, no? Not necessarily. What kind of wall are you hanging it on? Concrete, wood or old plaster that has the potential of crumbling to pieces? What if you are hanging a mirror with no frame? And just which nail or hook is best for the job — an S hook that hangs from molding, a three-pronged nail device that can hold up to 100 pounds, or maybe no nail at all but adhesive tape instead? It’s easy to get hung up fi guring out how best to hang up what you want. Here’s what’s out there, how best to use it, and what it costs: