Wine is a food product, constantly evolving and reacting to the external environment. Just like any food, the quality of a wine can be greatly affected, damaged or even ruined should it be improperly stored. In order to create the optimal conditions for a wine to best evolve, the temperature, humidity, light and stability (lack of vibrations) of the storage location must be monitored. Most wineries have historically stored their wines in underground cellars that naturally stabilize these factors over the course of the year, eliminating work on the part of the cellar master.

If a wine is released within a year or two of harvest, and the plan is to drink the wine shortly after purchase, you need not trouble yourself with expensive storage options. However for wines you choose to age, the cellar should take into account the following factors, in order of importance:

Nothing can damage wine more than temperature. If the temperature gets to be too high, the wine will spoil, taking on irreversible cooked flavors and have an accentuated alcohol taste. If the cellar or storage facility is too cold, it will slow down or even prevent the desired maturation of the wine. Ideally, the room should remain at a constant temperature of about 55ºF. A few degrees higher or lower are nothing to fret about. What should be avoided is a room where the temperature fluctuates on a regular basis as this will truly degrade the wine.
Maintaining a consistent temperature is particularly difficult if you are traveling. If you plan to carry wine back on a plane, do not put the wine in your checked luggage as there is no atmospheric control where the suitcases are stored and the baggage compartment's below freezing temperatures and pressure will damage the wine. If you are touring wine country by car, especially in warm climates or times of year, try not to leave the wine in the trunk . During the day, the wine will heat up considerably, and as night falls so will the temperature. Think about keeping a large cooler in your trunk for those weekend jaunts, or bringing the wine into your hotel room at night.
The ideal humidity for long-term wine storage is between 70 and 75%. To monitor the relative humidity in a cellar, a hygrometer can be purchased at some wine shops or at a hardware store. Humidity in the storage room prevents wine from evaporating too quickly from the bottle. Bottles should also be stored horizontally so as to keep wine in contact with the inner surface of the cork. And ensure that it doesn't dry out. On the other hand, humidity should not be too high because it could spur the growth of bacteria on the corks, ruining the wine.
In absence of a hygrometer, you can tell if your cellar is not humid enough by periodically checking the corks. Should you find the cork is wet on both ends and there is a visible reduction of wine in the neck of the bottle your wine is evaporating too quickly and you need a humidifier! A trick used in many old world cellars is covering the floor of your cellar with gravel or pebbles and sprinkling it from time to time with water. This will hold in the humidity, but it is smart to keep the bottles off the floor to prevent water damage or mold growth on the labels.
Though sunlight is among the most important things for the vine and grapes, it can be one of the most detrimental influences on wine in a bottle. Direct sunlight will not only heat up the wine, but when the sun is gone the the wine temperature will drop, causing dangerous fluctuations on a daily basis. Sunlight can also affect the taste of the wine and strip it of valuable elements. Florescent lights are also harmful to wine's aromas.
When buying wine in a store with florescent lights, reach for a bottle that is under a shelf or other bottles or at least not directly under the light source.

It is common knowledge among wine producers and collectors that wines prefer not to be disturbed. Most age-worthy red wines are highly tannic with more sediment that will eventually precipitate over time. It is best not to jostle the bottles so as to keep the wine stable. This is, for obvious reasons, even more crucial for sparkling wines. Of course if there is the potential for significant jostling, such as in an earthquake, the bottles should be padded to prevent breakage.