You need:
– something to stop it moving back under acceleration and wind resistence (tie the bow tightly forward).
– something to stop it moving forward under braking (a strap forward of the widest point).
– something to stop it moving side to side (a strap around the boat at the widest point: anchored to the trailer at the axle).
– enough padding (old squares of carpet upside down) to stop the ropes and straps rubbing.
– everything tightly tied so it doesn’t move
– an undercover to protect the hull from stone chips
– an over cover to stop it getting heavier when it rains.
– and of course a nice tow car to act as the support vehicle, as detailed here: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=5162&PN=1