The manner in which your wines are stored is extremely important if they are not to deteriorate early.
Ideally, they should be stored in a dark humid place with a constant temperature of around 53-55° F, lying on their sides with the the wine fully in contact with the cork. Conventional wisdom says that any air bubbles in the bottle should come into contact only with the glass. If a cellar is available, then this is probably the best place to try and approach these ideal conditions - providing there is nothing remotely resembling a central heating boiler located there. It is a constant mystery to me why, in our overcrowded island, where space for just about everything is at a premium, housebuilders do not provide cellars in newly built houses - in otherwise unused space that does not affect our visual landscape. Other European countries (eg Germany) do it, so why not us? Given that the ideal environment is not always available in a modern centrally-heated British house, it remains an ideal to which we should strive to come as close as possible.
To be avoided at all costs is temerature fluctuation - even if the ideal 53°-55° is not possible, you should at least try to ensure temperature stability. The absolute temperature affects evolution of the wine - the warmer it is, the faster it will develop. The wrong temperature can damage wines directly as well; if wines are too cold, they may freeze, and this can force the corks from the bottles, allowing wine to escape and oxygen to enter the bottles, oxidising the wine when it thaws. Too warm and the wine expands, also risking the cork being forced out, or wine escaping past the cork. Also excess heat can cause some of the more volatile compounds, which contribute directly to aroma and taste, to be lost.
Light is also a problem, and can affect the flavour of wines; this is why many wine bottles are dark in colour. Under no circumstances should wines be stored in direct sunlight, the heat of which can allow wine to seep past corks.
Humidity is also important, as the cork should not be allowed to dry out, as it may shrink, and the seal with the bottle and the wine may become imperfect, allowing oxidisation.
It is of course possible to get carried away with all this - being realistic, in a modern house, a good compromise is a wine rack in a cupboard under the stairs, where the temperature is reasonably constant in the range 10° to 20°C. This is fine for wines which are going to be stored over a fairly short period - say up to 3 or 4 years. Alternatives include special temperature-controlled cabinets, although these are expensive.
Before a bottle is opened, it is wise to stand it upright for several hours, or preferably overnight, to enable any sediment to settle to the bottom of the bottle. If the wine has thrown a heavy sediment, then you may wish to decant the wine. Natural sediments are quite harmless, and occur especially with wines that are unfiltered or lightly filtered. As to whether the bottle should be opened some time before drinking the wine, this depends on the wine itself.
Lighter fruity wines - like whites or lighter Beaujolais wines - do not really need time to breathe, and the fruity aromas and flavour will be evident as soon as it is opened. Older and heavier Crus, however, need some time to allow aeration of the wine before serving, as the flavour of a Cru Beaujolais can develop quite markedly over time it is exposed to air, initially improving, then fading. Allow an hour for a heavyweight Cru such as Moulin, Chénas, Juliénas, Morgon, with perhaps half an hour for the other Crus. Again this is a generalisation, but the best thing is to experiment for yourself. If you don't have time to allow the wine to breathe properly, the process can be accelerated by decanting, using a decanter in which the wine is exposed to a larger surface area in which to come in contact with air.
I have included in the tasting notes on our website wherever possible recommendations from the growers about the temperatures at which their own wines should be served. Generally speaking, the lighter Beaujolais wines should be served at somewhat lower temperatures than the heavier Crus, and as the wines age, the temperature at which they should be served rises. Also, stronger wines are served slightly warmer than lighter low-alcohol wines. These are of course just generalisations, and will not apply in every case; they may however be used where no other information is available.
Generally speaking, Beaujolais is a light fruity wine and can be served somewhat cooler than many other red wines. Chilling a light Beaujolais, Beaujolais Villages, or Chiroubles is quite acceptable, however the heavier wines from these appellations, and the other Crus, are intended to be drunk at or near room temperature, and if they are chilled then the aromas and flavour will be hidden. Perhaps 12-13° is appropriate for lighter Beaujolais, some Beaujolais Villages, and lighter Crus such as Chiroubles, rising to maybe 17° for mature heavyweights like Chénas and Moulin.
Always allow the bottle to reach the serving temperature naturally - leave it in the room in which it is to be served for some hours before consumption. Forced heating or cooling is to be avoided.
I was given a tip about the correct temperature at which a red Beaujolais should be served, by a vigneron for whom I have great respect. This is that where no specific guidance for a particular wine is available, a good thing to do is to take the alcoholic strength in %, and to serve at that temperature in °C. So a wine with 13% alcohol should be served at 13°C.
If you don't finish the bottle in a single sitting, it will oxidise if left for more than a day or so, so it is really best to drink it all the day it is opened. There are devices which extract oxygen from partially consumed bottles, and these generally work quite well in terms of their ability to prevent oxidation and prevent the wine from going off, however the aroma of the remaining wine will suffer.