Leftover Champagne may be a foreign concept to some, but if you entertain often, it's certainly a possibility. There's good news, though: It is possible to save a half-full bottle of bubbly for another day, without it going flat. If you're enjoying a bottle with friends, you don't have to worry about it losing its effervescence between rounds. Champagne will stay sparkling in the bottle for a few hours after opening, says Krug Brand Director Jamie Soriano. She recommends chilling a bottle of bubbly to 40 or 45 °F (and for Krug, a bit warmer at 55 °F, to help its aroma fully emerge) before opening. Then, while you're drinking the wine, keep the bottle over ice to maintain that temperature in between pours.
As with any wine, Champagne ages once it's opened. When a bottle sits out, exposed to air, "it is always evolving," thanks to that contact with oxygen, Soriano says. The longer Champagne is open and exposed to oxygen, the more bubbles it loses; the decrease of fizz gives the wine a richer mouthfeel, and, alas, a flatter texture.
So, what to do if you just don't have it in you to finish the bottle? Soriano, for one, admires your impressive willpower. Her advice is to invest in a special Champagne stopper to keep the contents of an opened bottle fresh. The difference between a Champagne stopper and a regular wine stopper is that the former contains a push bottle seal and lever function, which helps keep a tight seal and maintain the wine's effervescence. Once you've sealed it tightly, store the bottle in the refrigerator no more than a day or two to ensure freshness.
The real lesson here is to invite your neighbors over, extend the party an hour longer, and do whatever it takes so that no opened bottle of Champagne goes unfinished the same day you've opened it. Cheers!