"What's a good year?" I'm often asked about wine.
"Why?" I question.
"Because I want to buy some wine to lay down" I'm told.
"Well that all depends on why you want to lay it down. Do you want to create a cellar for yourself, or buy to re-sell as an investment, or lay down for a special occasion?" I ask.
"Errrrr not sure, I think for me, for later." The response.
"How much do you want to spend and what are you looking for?"
"Not sure, not much and I like everything." was the "helpful" reply

The conversations go on and on but "laying wine down" is at the tip of many people's tongue once into wine.
Laying down wine means to putting wine to one side in favourable conditions and allowing it to mature and improve with age. 
For most people the "laying down" occurs in their own cellars but others with large amounts of wine (and cash) use controlled cellars where companies cellar your wine in guaranteed controlled and insured conditions much like rented storage units.
I'm going to be discussing the former as I don’t think the latter is something your average wine drinker is particularly interested in.

So, how do you know what to lay down? Here are a couple of quick pointers:

First of all, ask questions about a wines longevity before purchasing.
It’s all well and good buying wine but if your buying the wrong wine you will either need to have a party to drink it while it’s ready or pour it down the sink because it’s past it’s prime.

Not all wines are cellarable.
A lot of wines are made to be drunk today and will be spoiled if they sit in a cellar. 

In general white wines have a shorter cellar life than reds (there are some exceptions).

Know your producer, do your homework.
Some producers are renowned for using new world techniques that can bring out the flavours in the wine making it drinkable at release while others are renowned for using old world techniques requiring the wine to be laid down before they are at their prime to drink.

Certain grape varieties such as Cabernet Sauvignon, Nebbiolo and Malbec lend themselves to long term cellaring due to their high tannin and acidity (tannins soften with age).
Others such as Pinot Noir, Gamay and Tempranillo with lighter tannin have a lesser cellaring timeframe as their delicate light tannins are quickly lost.

Just because it’s got a fancy label or comes from a big wine producing area doesn’t mean it’s going to taste any better after 3 years in your cellar, don’t be fooled by appearances.

By wine books or magazines that have specific producer, label and vintage information.
Often these will tell you if the wine is worth cellaring.

Here's a link to an interesting infographic by Wine Folly about what wines to cellar http://winefolly.com/tutorial/cellar-wine-guide/