How to waterproof D200 or best portable waterproof case?

I am going on a trip and am looking for the best, relatively inexpensive, way to waterproof my D200. If this is not possible then what is the best case. I have looked at pelican Cases, but they are gargantuan. I am looking for something that I can get easy access to the equipment. If it must be a bag then it must have enough space for me to pack some of my gear, such as layers of clothing. No food need be stored there, just warmer layers in case it gets cold. I will be hiking as well as going on the river. Any help would be appreciated.