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Pour your leftover wine into a half-size wine bottle and seal. This will reduce the surface area of the wine that's exposed to oxygen, slowing the aging process. Make sure your half bottle of leftover wine is sealed tightly with an appropriate cork, stopper, or screw-top. Save empty half bottles, which you can often find when buying dessert wines, and reuse over and over again for this purpose. If you don't have any half bottles on hand, you can use another small glass container that seals tightly. Buy a vacuum cap system for wine, which removes the oxygen from inside the bottle. Potentially lengthen the freshness of leftover wine in this way. You may want to invest in this device if you frequently have opened bottles of wine to keep, or drink varieties particularly prone to oxygenating, like full-bodied white wines such as oaked Chardonnay or Viognier. Note that there is some disagreement about the effectiveness of wine vacuums. Some say that the oxygen removal is only partial, or it can actually damage the flavor of the wine due to extracting its aromas as well as the oxygen. Replace the oxygen in an opened bottle of wine with an inert gas, most commonly Argon. You can buy a device for this purpose from wine retailers. Try an aerosol spray for an inexpensive option, or a more sophisticated system like the Coravin. Invest in this system if you're a wine connoisseur who frequently needs to keep open bottles, as in a restaurant or other serving setting.
Transfer to a half bottle. Purchase a vacuum pump. Invest in an inert gas system.