Sean's advice is on the money cell count-wise and storage-wise. Slurry is a temporary means of storage. The easiest, most compact, and most foolproof way to store yeast for more than a month or two is on slant. Now, if you can afford to operate a -80C freezer, cryostorage is the way to go. I own quite a few expensive cultures, but I cannot justify the cost of a -80C freezer.