buy a hardsided coffin, and do all the wrapping and padding the others above recommend. pack some suncure and a repair kit too. the airline will find a way to damage it somehow.
I just got back from Puerto Rico and took three boards in a finless coffin. Wraped the bottom one completely with bubble wrap, wraped the rails of the top two with bubble wrap, took the seperator that came with the bag and put it on top. It seemed like it was more than enough.