Lots of people automatically go to bungee-like tiedowns and cargo nets because that is what is commonly available in bike shops. I have found that nylon straps with buckles are much more secure and its easier to fasten all kinds of odd shaped loads securely with them. Straps are usually available for $6-$8 each at camping equipment stores or touring oriented websites.
Here are some examples:
http://www.rivbike.com/products/show...h-strap/20-138
http://www.rei.com/product/800862
I use the straps sold by Rivendell.