I usually open older bottles of wine, such as the Storybook Mountain Zinfandel from the 1980s (below) on occasions when there are enough dinner guests to ensure that the whole bottle is consumed on the same evening. No matter how well preserved the wine might be, there is nearly always deterioration on the second night. And on the third night, it is usually dead. I have tried various methods such as pumping air out of the bottle with a Vacu-Vin but have never been totally satisfied until I learned about the "repour" wine preserver.
The device is simply a wine stopper that is capable of removing the oxygen in the air above the wine. Pour yourself a glass (or more) of the wine and then immediately insert the stopper until you are ready to pour another glass. As the manufacturer claims: "Repour will keep your favorite wine fresh for days, weeks, or months!"
I haven't tried the method for a month; that claim seems hard to believe. But I tried it for a week with the Storybook Mountain Zinfandel with excellent results. Every glass has been fresh and well preserved: full bodied, rich in fruit with enough acidity to invite another taste (but not enough to create a harsh edge).
Each repour stopper works for one bottle only, but the price is reasonable--10 stoppers for less than $20. If it really keeps a wine fresh for months, then this method is comparable to what you might get with a much more expensive Coravin wine preserver.