If you don't want to finish a bottle there is no need to think it will necessarily go to waste. There are plenty of ways of protecting the wine in an unfinished bottle once you understand a little about the science of wine. Wine is a fragile liquid mix of chemicals (don't worry, most of them are natural) and the mixture changes on contact with air, or specifically with the oxygen in the air. At first after opening a bottle the effect of air can be beneficial to the wine, which is why it is advised to open some wines a little in advance of serving, or better still to decant the wine when it will be 'mixed' with oxygen more quickly. However, eventually the oxygen will have a detrimental effect on the wine, initially meaning the wine loses its fruit and freshness, and later changing the composition in the wine so much that it becomes oxidized, lake a bad, brown apple. Sometimes, this is termed Maderized or 'like Madeira', though technically this is slightly different.
The most important preservation method for an opened bottle of wine is cool temperature. Especially in summer, it's best to keep all opened wine in the fridge with the cork put back in, or the screw cap back on. If the wine is red, it will soon warm up once brought out of the fridge. Both white and red wines spoil equally quickly, by the way.
Real wine aficionados who are modest drinkers keep a stack of half-bottles handy. Then if they are opening a particularly fine bottle, they quickly decant what they know they won't drink into a half bottle and that way the half will be full, and will last much better than keeping a full bottle half-full!
Other methods involve purchasing either a vacuum pump system (vacuvin is a popular brand of pump and stoppers) or an inert gas system. The vacuum pump works by pumping out the air, leaving a vacuum on top of the wine, but does risk pumping out some of the flavour compounds. It's not too bad on a cheap to mid-price wine, especially if you only pump the wine once. If you do it after every glass, the wine will soon deteriorate.
The inert gas method replicates a system used in many wineries, where tanks that are not completely filled have a layer of inert gas (Nitrogen, Carbon Dioxide, Argon or any combination of the three) pumped into the tank. There are a few brands of canisters sold, usually filled with argon that are ideal for home use. The canisters seem like they weigh nothing and one only needs a quick squirt into the bottle and the heavier inert gas will displace the oxygen on top of the wine. The canisters usually last for several hundred bottles and don't cost much.
Many pubs and bars use wine boxes instead of bottles for their everyday wines, and these work for home use too. They work by having a collapsible bag inside the box, thus excluding air each time an amount of wine is drawn off from the box.
Remember, the more wine that is taken out of the bottle, the more fragile the wine will be. It's often better to cook with what's left in a bottle if there's only the volume of a small glass or less left in it.
