Here's a basic but invaluable buying tip; for any given region it's worth concentrating on lesser wines in the best vintages, but stick to the known performers in the not so good years. In the worst years you should simply stay away.
It's simple really, the top estates have the resources to ensure attention is given to every minute detail of the grape growing and wine making process, taking all the necessary steps in lesser years to still come up with excellent wines. They also shouldn't be prohibitively expensive in these years (superstar wines excepted) which they will be in the best vintages. However, the more modest producers are more likely to succumb to the potential downfalls of a lesser vintage, whatever they might be. But in really good years all competent producers should manage to make decent wine.
Looking at claret for example (ie red wine from Bordeaux), assuming the huge hype around the 2009 vintage is justified, then the Petits Chateaux should manage to make terrific wines which should be well worth buying. Whereas in an average year, like 2004, it's worth trading up a bit, maybe buying one case from a leading producer and a couple from mid-level houses for example. In a poor year like 2007, there's no real point in buying at all.
If you are building up a cellar, modest or otherwise, following this simple rule should help to ensure you have a good range of quality wine and also give you a large drinking window, with some wines for more immediate drinking and others for the long haul.