Battery University

All you need to know about all types of batteries.

There are three pages of links and extra articles. Here is one popular newer article - How to increase the run time of your cell phone, although it is really designed to sell their equipment.

The big thing to take from the articles is that the new lithium batteries used in devices like cell phones are not like the old nickel-cads. For the new batteries charge them often and when they are not very drained. And remember to avoid getting your cell phone hot or wet.