How-To: Keep the Fizz in an Open Bottle of Champagne
While dining in Paris, Dorie Greenspan saw her waiter put a silver spoon in an opened bottle of Champagne before storing it in the refrigerator.
Sticking a silver spoon in an opened bottle of Champagne to preserve the fizz is an old wives' tale, but at least one waiter in Paris (among others) swears by this trick. Research has found (of course there's been research) that spoons don't do a whole lot but are better than recorking, that neither spoons nor corks are needed, as long as the bottle is refrigerated after opening, and that the only way to avoid losing fizz is to drink the whole bottle at once or seal it with a hermetic cork.
One of my friends explained that the spoon acts as a heat sink, thus cooling the inside of the bottle and causing a slower release of carbon dioxide.
Or maybe it's just urban mythology.