Li-Ion batteries start to degrade as soon as they leave the factory. There is nothing you can do to prevent it either. Keeping the battery fully charged will make it degrade quicker. Keeping the battery warm, like in a running laptop, will also speed its demise. This degradation shows up as a gradual shrinking of the battery's capacity.
Wiki has some information and links. There is a paper here that discusses it as well.
The battery also has circuitry in it that monitors the health of the battery. This benifits from being able to "re-learn" the capicity of the battery by occasionaly running the battery through a complete charge/discharge cycle.
So the best way to keep your laptop battery healthy is to keep it about 50% charged, in the fridge, and occasionally get it out, run it down, charge it completely, then run it back down to 50% before putting it back in the fridge.
You shouldn't store a laptop battery in a very low state of charge, becuase the battery contains electronics that monitor the health of the battery which use up power. If the battery charge gets too low then it becomes unsafe to charge it and the circuitry will kill the battery to prevent you from charging it.