Without knowing your bike's frame design it's hard to suggest a repeatable method. Just throw your bike onto the raft and figure ways to tighten it down via the attachment points. I used a bunch of toe straps and two adjustable length bungee cords. It was a bit tricky figuring out an arrangement that kept the fork high above the water line.
Here are also some ideas http://www.youtube.com/user/alpackar...?feature=watch