I got myself a couple of 1-liter Platypus "bottles" (see
http://www.platypushydration.com/pro....aspx?ProdID=2 , half the price they are here in England). When water is readily available, they roll up to nothing. In use, not being a fixed shape, they'll fit virtually anywhere depending on the amount of water you fill them with. There's all sorts of extras you can get and the bottles come in various sizes. I usually picked up a Gatorade wide bottle (or two) along the way, refilling the water bottles as required. Made sure there was always something to fall back on.