This is another good idea I stole from the US Military, and the SAS. As a kid going out into the woods on the various trips I went on, I was always bummed when the rain came during the day due to the contents of my pack usually getting soaked. I had an old german army pack made out of this rubber type of material that my I bought at the surplus when it was in Damascus. It was SUPPOSED to be water proof. Turns out, it was water tight! It would hold the rain water better than my canteen, and before I knew it, everything in my pack was soaked. When I was like 17 I worked for a guided fishing and hunting outfit called Wild River Ranch up in Eastern Oregon near Spray. We did guided fishing and hunting off the John Day River, anywhere from 1/2 day floats to full weeks going almost all the way down to the mouth of the John Day at the Columbia River. I bought a water proof sack for my things, it's a Momentum brand dry sack. It's made of some rubber/poly material, and you roll the top closed and snap it to secure it. After hearing how the military does it, I started using that sack in my pack as a pack liner to keep all my gear dry and it worked perfectly. Not only are they good to keep your gear dry in your pack from rain, but if you fall or jump into water with your pack on, or have to cross a river, your things will stay dry, and will actually act as a floatation device as they are air tight.