Wine gas: Inert gas will keep your wine fresh for days. (Inert gas is essentially pure air that contains no oxygen, and it prevents oxidation of the wine.) The two most common brands, Wine Life and Private Reserve, come in cans that feel empty. But really the gas inside is lighter than air and replaces any oxygen left in the bottle with the gas, preserving the wine. Just spray and re-cork and the wine will stay fresh for at least a few days and up to a week (and some say up to a month). You can find both brands in wine shops and online for around $10 a bottle; one bottle should be good for about one hundred uses. If you want be fancy about it, there is a “wine preservation steward” that replaces the oxygen with argon gas that sells for around $200 and keeps wine fresh for up to three weeks.
Wine vacuum: A vacuum pump draws out any oxygen in the bottle and seals it, preserving the wine for 2 to 3 weeks. You just place a stopper over the top and pump out the oxygen. They cost anywhere from $10 to $50. (But a vacuum won’t work on sparkling wines.)
Half bottles: An easy and cheap way to preserve your wine is to pour it in half bottles (375mL) and recork or use a wine stopper. The size of the bottle eliminates excess oxygen. Our winemaker friend swears by it. (For even fresher wine store the half bottle in the ‘fridge; see below.)
The refrigerator: Several winemakers I know stick recorked, half-empty bottles of wine in the fridge, both whites and reds. It slows down the deterioration of the wine. Just pull reds out of the fridge about an hour before you want to finish the bottle.
Bubbly: Obviously Champagne and sparkling wine pose additional problems given the CO2 involved (it’s impossible to preserve all those bubbles), but an inexpensive Champagne stopper will keep it overnight. Any longer than that just cook with it; leftover Champagne makes a great white sauce for chicken, fish, or pork.
Screwcap: If the bottle has a screwcap instead of a cork, you're one step ahead. A good screwcap actually preserves the wine better than a cork ever could. So once you unscrew that top, just screw it back on tightly (and even better, stick it in the ‘fridge) and enjoy the wine for up to a week.