Now some people may have already heard of these, but a friend of mine on the Texas gulf coast used hers after one of their hurricanes and had nothing but good to say about it, so I got myself one.
It's called a WaterBOB. It's a big bag you put in the bathtub and fill up with water, and it keeps the water clean and drinkable (and from leaking out the drain) for weeks. It's even got a hand pump built into it. It's single use so I haven't put mine to the test yet, but you can't have too much clean water on hand in an emergency.