Why do we decant?
When you decant a bottle of wine, several things happen. First, slow and careful decanting allows wine (particularly older wine) to separate from its sediment. Second, free sulfites, the preservative, will begin to dissipate from the wine. Free sulfites deter bacteria from multiplying and turning any wine into a highly unpalatable liquid.
Once a bottle is decanted, the shelf life is reduced to just a day or two.
How to Decant
Decanting a young wine (one with no sediment) is easy: Just pour it into the decanter. Let it sit for twenty minutes or so before you serve it, and you'll likely notice a dramatic increase in subtlety and complexity. Decanting wine with sediments requires a bit more finesse. One of the easiest procedures is to stand the wine bottle upright for a few days before opening it, so that all the sediment collects at the bottom. Simply pour slowly into a decanter, stopping before any sediment enters the decanter.
To decant a bottle straight out of your wine rack, you'll need two pieces of equipment: a light source (either a candle or a small flashlight) and a wine cradle.
Gently remove the bottle from the rack, keeping it horizontal, and place the bottle into the cradle so that it's just shy of horizontal (about a twenty degree angle). Now for the difficult step; remove the seal and cork without agitating the wine. Turn on your light source. Gently pick up the cradle and position the light behind the neck. Begin lifting the back of the cradle slowly to pour the wine into the decanter. Keep the light shining on the neck, and watch for sediment. When you get toward the end of the bottle, you'll start to see sediment creep up toward the neck. Stop pouring as soon as that happens. The wine you've just decanted will be clean and clear, and the sediment will be left in the bottle.
Take the wine left in the bottle (usually about a glass worth) and strain it into a separate container, using cheesecloth or an unbleached coffee filter. There are Wine Connoisseurs who claim the two resulting decanting jars have different tastes!
Cleaning a Decanter
It's always best to rinse decanters and stemware immediately after use. Never clean your decanter (or stemware) with detergent, because the shape of a decanter makes it very difficult to get the soapy residue out. Instead, use a mixture of crushed ice and coarse salt -- they'll remove any residual wine without leaving behind any aroma of their own.