Kindle uses Li-ion polymer batteries. For maximum battery life, leave it in airplane mode except when you need to connect to the internet. In normal use, recharge it regularly by topping off every few days and try to maintain your charge level between 30% - 80%. In normal use, try not to let the battery drain completely. If you aren't using it, you should still try to recharge it every few weeks.
Unlike older ni-cads, li-ion batteries don't have a memory effect and don't require charging cycles, although there's some support for the idea that completely discharging them every month or so may be of some benefit. I don't bother trying to do this and have never found it worth the trouble. With normal use, the average life of Li-ion batteries is about 3 to 5 years.
This article contains excellent information about best practices to maintain Li Ion batteries:
http://lifehacker.com/5875162/how-of...g-its-lifespan
Complete information about Li-ion and other rechargeable batteries can be found at Battery University:
http://batteryuniversity.com/learn/a..._ion_batteries