Most efficient way to clone an object?

here doesn't seem to be an in-built one, you could try:

function clone(obj){
    if(obj == null || typeof(obj) != 'object')
        return obj;

    var temp = obj.constructor(); // changed

    for(var key in obj)
        temp[key] = clone(obj[key]);
    return temp;
}

There's a lengthy post with many contributing comments on Keith Deven's blog.
If you want to stick to a framework, JQuery also has a clone() function:

// Clone current element
var cloned = $(this).clone();