You simply must decant

Broadly speaking, there are 3 reasons to decant wine before serving...

1. To remove the wine from the sediment - as wine ages chemical changes take place, some of which result in sediment at the bottom - and you just don't want this stuff in your glass.
2. To let the wine breathe - more below.
3. To let some bad aromas "blow off" - some wines can actually smell quite bad when you first open them - decanting helps these bad smells to dissipate.

I think that people have the wrong idea about decanting - that for some reason it's regarded as some kind of high-class snooty thing that fancy restaurants do, or something you only do with an expensive, old wine.

Wrong!

The simple fact is that just about every wine tastes better if you've given it some time to breathe - to react with the air and "open up" - yes, even 5€ wines improve if you give them some time before drinking.

If you don't believe me - try it. Pop your cork and carefully pour the wine into a decanter (or jug if you don't have one) - stop when you see in the neck of the bottle that the liquid is no longer clear. Pour a little into a glass, swirl it around a bit, sniff it, and taste it - concentrate on the sensations. Then wait a few hours and do the same - very often you'll find that it's a totally different wine - a better wine.

A couple of weeks ago it was my privilege to drink a bottle - my last - of Avó Sabica 2004. I popped the cork 3 hours before dinner - the wine tasted of absolutely nothing - I sipped again - nothing - I checked with my wife - nothing. But after those 3 hours it had changed completely into an amazingly beautiful wine.

"Can't I just open the bottle and let it sit for a while"?

No.

Look at the top of the bottle - there's almost no contact between wine and air, so almost no breathing takes place. On the contrary, in a decanter there's a lot of wine to air contact - lots of breathing.

You'll see other people recommend this method - they'll call it "slow-ox" (slow oxidation). My experience is that this simply doesn't work for wine or port, but, rather strangely, works better for Moscatel than decanting.

"I got home late and didn't have time to decant."

Of course, in this situation you will have picked a cheaper wine. Splash the wine into the decanter - you're trying to get as much air to wine contact as possible. If you have a funnel to hand then, having washed out the bottle, pour it back and decant it again. Whilst not ideal, this can certainly help improve the wine and will be 5 minutes well spent.

"Surely I've got to have decided what I'm going to drink ahead of time"?

Errr, yes - no way around that.

"How long should I decant for"?

Now there's a good question. My default is 3 hours for red wine, and at least 8 hours for port - I'll give young vintage port 18 or 24 hours - it really does make a huge difference. I tend to give "bigger" or "heavier" wines a little more time, and "older" wines (20 years+) a bit less. Very tannic grape varieties like Baga or Ramisco a bit longer. I think Syrah likes a long decant.

If you assume that it will take you a couple of hours to finish the bottle, try to concentrate on that last glass - is it better now than it was when you started? If so, try to remember (or, better yet, write a tasting note), and when you have the same wine again give it more time.

"I only paid 2€ for this bottle, is it really worth the bother"?

Absolutely - if you can make a 2€ wine taste like a 4€ wine you've doubled its value!

"What about in restaurants"?

This really depends on how classy the restaurant is, and how good their wine list - any restaurant expecting you to pay 200+ € for a Barca-Velha or Pêra-Manca ought to expect to serve it in the very best way possible - carefully decanted. Ask before you order - if they can't decant it, just don't buy it! I personally wouldn't pay over 30€ for a wine that a restaurant won't decant.

What isn't on your side in a restaurant is time - you're unlikely to want to wait 3 hours before you start eating, and the waiters probably wouldn't be that happy either. After the wine is decanted, take a very small glass - swirl it around a lot - sip slowly until the wine has had at least some time to open up - swirl the decanter from time to time as well since that will help.

"What kind of decanter should I buy"?
Excellent question and one I'm so very pleased you asked.
Obviously there are issues of style and taste here, not to mention cost. But what you shouldn't forget is that "Form Follows Function" - the main aim of a decanter is to air the wine so, no matter how beautiful (or expensive) the decanter, if it isn't doing that then it isn't doing the job, and your wine isn't getting the air it needs to be the best it can.
And the key here is for the wine to have significant contact with the air.
For a long time I used this most beautiful decanter from VistaAlegre...

I proudly showed a picture of my wine in this to a drinking friend of mine (doffs hat to André over in Cologne), who pointed out to me that, since the wine filled most of the volume of the thing, I actually wasn't getting very much wine to air contact - in short, that it wasn't working very well.
But I very much loved this decanter (and still do), so I resisted. He then suggested I buy a cheapie decanter and run a test, which I did. I decanted half a bottle from a Magnum (1.5l = 2 bottles) into my beautiful decanter, and the other half into a decanter I bought, for about 5€, in the local supermarket.
After the requisite 3 hours both my wife and I each did a "blind tasting" - comparing samples from both the decanters, without knowing which was which.
We both agreed that the wine tasted noticably better from the cheapie, which looked something like this...

With a bottle of wine in this beastie, it comes up to the widest part of the bowl, providing a lot of air to wine contact, which is what the doctor orders.
I've bought a few decanters since then - always with a principle focus on their function.

I still sometimes use my beautiful decanter - if we have guests I might move the wine from a cheapie into it, or possibly for a bottle of port I'm expecting to last a number of days (although that's rare).


"I want to take some good wine to friend's for dinner"
No problem - use your decanter as if you were going to drink the wine at home - just before you leave, pour the wine back into the original bottle, which you've washed out and re-insert the cork, or use one of the wine-bottle closure gizmos. This is sometimes called "double-decanting".