When you're dealing with fragile items, sometimes just using one box won't provide enough protection. To provide double the protection, try using two boxes by nesting a box within a box.
To start, wrap about two inches of cushioning around your item and place your cushioned item into a box. Use peanuts or some other cushioning material to restrict the movement of the item within the box, with a minimum of two inches thickness on each side. Restricting movement is very important so your item won't get broken or damaged.
Place the first box into a second box at least six inches longer, wider, and higher than the first. Use the same cushioning material that you did with the first to keep the smaller box from moving around too much in the larger one.