Wine is best stored between 40-65°F (4-18°C), optimally between 55-57°F (13-14°C). Even more important is temperature stability. Optimally the temperature in the cellar shouldn't fluctuate more than 5°F (2-3°c) in a year. This keeps oxygen from being drawn into the bottle, upsetting the freshness or even spoiling the wine. Maintaining this sort of stability can be hard to achieve, so simply do the best you can. A Max-Min thermometer will give you a good indicator of the temperature swings in your storage area over time and many will also measure humidity.
Also, keep the wine at a humidity above 50% and below 80% as this is enough to keep your cork from breaking down but dry enough to keep mildew from growing on the cork or label.
It's a good idea to keep the storage area somewhat ventilated and odor-free. Air will inevitably pass into your wine so the odorous matter in the air will make its way into your wine. A shallow pan of water and a fan are great options for acheiving both humidity and ventilation goals.
Finally, wine should be kept out of sunlight. as the light waves can breakdown the more complex particles in your wine. Packaging your wine in dark bottles (green or brown) will also help absorb harmful light. You also need to note what type of cork you are using, as natural corks need to be stored on their side to avoid the cork drying out.
For more information, check out our PDF titled "What do I want to do if I plan on storing my wine for several years?"