The main factors I have found with longer term storage is the rigor of sanitization, amount of oxygen exposure during later fermentation and bottling, and the style of beer
Obviously, if you have some potential for contamination, this will slowly degrade the beer. Most often something like pediococcus can easily take over and introduce off flavors (buttery (diactyl), souring).
The amount of oxygen due to splashing, open containers, excessive mixing will cause early staling, loss of clarity.
Lastly, style of beer has a lot to contribute to shelf life. IPAs and other hop prominent styles will lose some of the aroma and bitterness as the hop oils fall out of solution with the proteins left in the beer. Malty styles seem to last much longer without perceptible degradation of flavors. Higher alcohol styles also help with retarding the growth of any bacteria or wild yeasts which may have snuck into the beer.
To get the best shelf life, I have found that storage at stable, cooler temperatures goes a long way. Cooler temperatures reduce the rate of oxygenation effects. Stable temperature of storage prevents potential leakage of air into the bottles as temperature fluctuations cause expansion and shrinkage inside the bottle.
I have scotch ales that have been in storage for 3 years now (I just finished off the last bottle of my batch from 4 years ago last month) and still seem to be as fresh and clean as the last batch I bottled two months ago. My storage temperature is not great as I don't have room in my several refrigerators for storing a lot of beer, but it sits in a corner of my basement that has low air flow and runs from about 55F right now up to a temperature of around 60F in the heat of summer. I typically brew and store Scotch ale, Doppelbock, and strong Belgian styles for storage. One of these days I'll have a chance to brew a barleywine for storage.
Another option to consider is to brew smaller batches. The bast majority of my batches are 10 liters into the fermentor. I yield a bottle or two over a case. Of course I make up for that by brewing more often so I still end up with a good full basement by the time I need to stop come spring.
