It happens. You open and bottle of wine but don't finish it. Now what? Well, you a have a couple of options.
First, it's best if you can limit wine's exposure to air. Exposure to air will tend to take the life out of it. So, it's best to use a device that can remove the air from the bottle and seal it. There are several such inexpensive devices that will do this. They consist of bottle stoppers and a vacuum pump. After placing the rubber stopper in the bottle, you simply hand-pump the bottle to remove the air. First issue resolved!
The second thing you should do is to keep the wine cool. Any kind of heat of 65 degrees F or higher will also ruin a wine. If it's a white wine, simply put it in the refrigerator. It will keep nicely for 1-3 days. When you're ready to finish the bottle, just take it out of the refrigerator about 30 minutes before you intend to drink it. That way it'll be at the right temperature. If it's a red wine, you can also put it in the refrigerator or simply store it in a nice cool spot that gets little or no sun exposure. In the refrigerator, a red is probably only going to hold up well overnight. So, be sure to enjoy the rest of the wine the next day.
Of course, the easiest way to avoid the problem all together is to finish the bottle! But when that's not an option, simply try to limit the open bottle's exposure to air and keep it cool. Then finish the bottle soon. Cheers!