I RARELY SAVE my unfinished bottles of wine. Few wines are as good, let alone better, on the second or third day as they are on the first.

I’m more likely to give away half-finished bottles to my friends and neighbors than I am to squirrel them away for another day—although, to be honest, there’s never a lot of unfinished wine in our house.

I realize that I’m very much in the minority. Almost everyone I know seems to have several half-drunk bottles stored on their kitchen counters or inside their refrigerator doors.

There are two basic ways to keep an open bottle of wine fresh and protect it against oxidation (air is the enemy of wine, after all): Either the air in the bottle is pumped out or the remaining wine is covered with a protective blanket of gas. The simplest air-pump system is a small plastic device that allows wine drinkers to remove air from the bottle by hand. Vacu Vin’s Wine Saver, which debuted 30 years ago, is probably the best known. The simplest—and cheapest—form of protective gas comes in a can and is sprayed directly into the bottle.