I forgot to mention something very useful that we used on our motorcycle trip to California – Pannier or Saddlebag liners. We purchased these in the UK before we left and took them with us. As we were renting bikes, we wanted to be able to put our belongings in a bag that we could lift out when we arrived at the hotel for the night. We each had 3 and everything we took went in these.
I have to say they were excellent – waterproof with a top that sealed and this formed a carrying strap. I would definitely recommend these for anybody going touring and renting a bike.
They are called Exped Shrink Bags, come in different sizes and are available in the UK from http://www.gearpac.com