Yakima makes rack mounts called Hully Rollers that are rollers that attach to a rack. This allows the boat to be rolled on from the rear if that is feasable from your vehicle. Also there is a system that will lower rods down the sid and pull the boat up on top; failry pricy but other wise looks good. Finally those round foam pool toys, the ones at Wall-Mart that float, can be used to roll a boat up from the side.
Good luck.