Before choosing a space be sure it will be large enough to accommodate future purchases. In some cases, vacant space beneath a stairway is sufficient, in others it may be necessary to allow for hundreds of wines stored both as individually racked bottles and full cases. One of wine's greatest enemies is extreme heat. Equally important is the rate at which temperature changes. Rapid temperature fluctuations may cause pressure changes within a bottle, forcing the cork upwards and allowing leaks while permitting air to enter the bottle.
Air is another of wine's enemies.
Any prolonged exposure will lead to oxidation, which produces a brownish color and Sherry-like flavors. Therefore it is important to have a cool space with constant temperature for long-term storage. Light may also harm wine over time. Bottles should be kept from direct sunlight, preferably in darkness, and should be stored on their sides, either in cases or racked. Humidity is more controversial but it's nothing to worry about unless you're keeping wines for the long-term.