Most people on OB have a more polished solutions than I do. I have some old milk jug containers from Costco stored in our fridge that I cleaned out thoroughly and regularly drain and refill (I call them our "earthquake water" for emergencies and they are more of a brick shape than a regular milk jug so they fit in a cooler much better). A couple days before a trip, we take two of these, freeze them and put them in our cooler instead of ice. They'll last for several days keeping everything cold and, once they melt a little bit, there's nothing more refreshing than ice cold water after a long hike. Until they melt though, we usually bring two more jugs of non-frozen water for drinking and we fill our hydration packs before we leave. This gives us a total of ~5+ gallons for two people (2 frozen, 2 non-frozen, +4L in our hydration packs and any water bottles that we brought along).
Typically it's just my girlfriend and me that go on trips so it's easier to figure out supplies (plus, we have a small Jeep so space trades at a premium). Generally we just do weekend trips and, on longer trips, we are able to refill our containers mid-trip. My philosophy is that less money spent on water containers = more money to buy gas and parts to keep the Jeep running.
