I know we all bash FEMA quite a bit, but here's a link to their suggestions:
http://www.fema.gov/plan/prepare/water.shtm

Doug's got great information here on ETS:
http://www.equipped.org/72hourkit.htm

Both recommend a gallon per person, per day. FEMA goes on to mention that a normally active person needs at least a 1/2 gallon per day just for drinking.

I toyed with keeping water in a large Reliance water jug and rotating it every month. That became a real pain in the you-know-what. Then I tried the Nalgene bottles and Platypus water bladder route. Finally I came to realize simplicity is great.

I went with mostly 0.5L bottled water. Sure, it can get somewhat expensive. But I try to buy in bulk from Costco, Wal-Mart (ugh), or on sale at the grocery store. I've found that not only is it easier to rotate, I also drink more water day to day, which is a good thing. Especially in the hot summer. Plus, with the half-liter bottles you can easily hand them out, throw them in the cooler while camping, put in your backpack for a quick hike (yeah, I know, not really leak-resistant. Nalgene's rock!), or whatever else. They also keep nice and cold in the fridge.

Also, I've picked up a few of the 1 gallon Arrowhead jugs at Wal-Mart that are all of $1 each. They make a good augmentation. For $3 you can have enough water for 72 hours for 1 person. Not bad at all.