
Avoid heat, first and foremost. Wine should typically be stored at 55 degrees Fahrenheit. Extremes in temperature can damage and prematurely age the wine, reducing its shelf life considerably. More importantly than the temperature itself, many studies have shown that spikes and huge swings in temperature are dangerous, even for comparatively short periods of time. This is why it is best to have your wine shipped in the fall/spring if you can, or cough up the extra cash for overnight shipping if you have to ship during the extreme months of summer and winter.
Wine with cork closures should be laid down on its side, this will keep the cork moist by having it in contact with the wine itself.
Avoid direct light, as the effects of prolonged exposure to light are well documented and can break down a wine rather quickly.
Humidity: most agree on a measure of humidity, say 80 percent. This is solely for the life of the cork. If a cork dries out, it shrinks. If it shrinks, it loses its seal. If the seal is broken, then air can enter the bottle, oxidizing the wine. Bad things happen when wine and oxygen get to spend a whole lot of time together. The baddest thing that happens is called vinegar. Great for salads, bad for wine.
So to recap, keep you wine in a cool, damp, dark and horizontal space. It is no wonder that old stone cellars and cave dug into the sides of mountains have been effectively used to store wine for centuries.