Li-ion batteries lose capacity by aging. The chemical processes related to this process run faster with elevated temperature and higher charge level. For maximum cell life, store the cells half full in a cool place (like a cellar or fridge) when you don't use them for extended periods. See this link for more info. We use high quality brand cells which have a much longer life than cheap no-name ones, they should last for years!

Now you know why laptop batteries die so fast: They are always full (when running the laptop from mains power) and always hot... batteries last less than a year under these conditions.