For each of my laptops, I use two batteries: a high-capacity one that I use when traveling and a regular one for daily use. When unused, I store the high-capacity battery (charged to about 40%) in the fridge, wrapped in a plastic bag. Thanks to this treatment, the 9-cell battery of my old spare laptop (Dell D430) is still good for 4-5 hours of use. It lasted for 6 hours when I bought in 2008.