Double-box it with plenty of packing material. Here's what I've done:
Wrap in plastic (a large garbage bag will do), then put in first box, with enough packing (large-cell bubble wrap is best; crumpled newspaper will do, if necessary) around it to immobilize the instrument. Then put this box inside a larger box, again using packing material to cushion & hold the inner box firmly in place. If you dont have the long, flat boxes needed to do this correctly, try checking with a local guitar store to see if they have some "empties" you could have....