I remembered this the next time I found myself with a leftover bottle of beer, and tried sealing it with an old wine cork. The next day, the beer was slightly less bubbly, but still frothy, flavorful, and most certainly drinkable.
I've since come to rely on rubber stoppers and a vacuum pump — the same I use for preserving a half-full bottle of wine. Once secured with the vacuum sealer, these stoppers keep an airtight seal and seem to work very well at holding CO2 in the bottle. (Careful, though: if the bottle is still more than three-quarters full, it can sometimes froth up when you pump the vacuum sealer.)
Sealed and refrigerated, I find that a bottle of beer will keep fairly well for about three days. After that point, the beer starts to taste too flat to be enjoyable to drink.
• Wine Bottle Vacuum Sealer Pump with 2 Stoppers, $4.20 from Amazon.com
(Image: Emma Christensen)