Stitchnripper's link is a good one and I recommend it. If your machine must be shipped, double-boxing is the way you should go. Use fiberglass strapping tape to secure the box. I have used an entire roll of it to seal one box. Start the roll of tape on one corner of the box (I usually start at the top left, about 3" down from the corner) and go diagonally in a straight line to the other corner. Do not cut the tape at any point - just keep rolling around and around wherever the tape takes you. This will preserve the structural integrity of the box.
At my most recent packing experience, I packed some boxes and a coworker packed others. There was much laughter as I rolled and rollled tape around my boxes. All boxes were transferred by the same shipper. Mine arrived totally intact, contents untouched. Boxes packed by my coworker all arrived damaged, some with contents spilling out. For the cost of a roll of tape, it's a no-brainer.