I've done a bit of late season hunting out of the truck, but never backpacking. Generally when car camping I leave a lot of water jugs in the truck, and so far that has been good enough to prevent it from freezing over night. I've never backpacked in really cold temperatures but I expect to do so this year. So I'm not really sure what I need to do to prevent water from freezing over night. Any tips? Gear recommendations?