Exposure to air causes wine to age. If you don't finish a bottle of wine, cork it up tightly to preserve what's left. The less wine left in the bottle, the more air, and the faster the wine will oxidize and lose its freshness. One strategy to maximize longevity of an unfinished bottle of wine is to transfer it to a smaller bottle -- say, from a standard 750 ml. bottle to a clean 375 ml. half-bottle.
Unfinished white wines, tightly corked and refrigerated, should maintain their character for up to five days, while reds will begin to degrade after 48 hours. (You can extend this slightly by refrigerating reds too, but then you'll have to let them warm to room temperature before serving.)
There are other ways to extend the life of wines after opening, all of which involve purging the bottle of oxygen. Two inexpensive devices are the Vacu-Vin®, which pumps air from the bottle and seals it with a rubber gasket and Private Preserve®, a canister of nitrogen you spray into the bottle to displace the oxygen. Both will extend the life of the wine an extra few days.
