I'd stay storing it in an airtight plastic container in the fridge and in water - for the portions not used? But you have to change the water daily and tofu only stores about a week or so - if soft. Pressed bricks may store longer in the fridge.
From: How to Buy and Store Tofu | eHow.com
From: How to Buy and Store Tofu | eHow.com
Extra info about tofu for you: Silken Tofu and Regular Tofu - Different Types of Tofu - Kinds of Tofu - Soft tofu - Firm tofu - Silken tofu