Ya know, I just gotta say for those that tend toward bungee cables...
It CAN be done, and it can be done properly, with everything secure and no wear and tear on the bike and without the extra expenditure of unnecessary nets, etc...
BUT, you gotta be able to tie down as if your life depends on it and it takes some elbow grease. I wish I had pictures of my first few trips.
Backpack, large garbage bag, 5 varying length bungees, and some soft rags.
Use the rags to wrap around the bungees where they may contact the bike. Use only secure, preferably frame parts or rear pegs to attach cables to, and go not only left to right, but side to side, intertwining and essentially making your own net. YOu can also hook bungees under the rear cowl or seat and put it back on for addtl security. Just a thought for those who dont have the cash to buy fancy stuff or bags, etc...you an also just take the seat or cowl off for a flatter more stable bag position.