Susan Hohlweg posted this method on Prodgy a few months back:
Put a healthy knob of fresh ginger in a large pot of good
soil. Keep it moist, not wet and in a sunny place. After
about a month, you will be able to enjoy the bonus of a
up-to-4 foot high plant. When digging up the root, you
will find it has many knobs (up to eight). Replant one
knob, to regrow. Enjoy the rest as your new plant
I found it worked best if you used the whole large piece
and had it at room temperature when you planted it.
This comment was added by Karen Ruedebusch:
I leaned this neat trick years ago from Filipino
friends. They just leave it in the the soil until they need
some, then take it out, cut off what they need, and return
it to the soil. You don't have to worry about how to store
it that way.