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On newer Android devices it is not recommended to use task killers. Android 2.2, for example, has better memory management than previous versions of Android. The OS is now aware of the memory usage of processes running under it... and is able to prune memory from those inactive apps when needed. Automatic task killers can be challenging because they go out and stop processes sitting dormant in memory. And they can be troublesome when they kill processes that Android thinks are necessary. The task killer kills the process, then Android restarts it, then the task killer kills it again, and Android restarts it again, etc.
To improve battery life go through your phones home screens and delete any widgets that you don't regularly use. Keep in mind that widgets runs all the time, whether you use them or not. Also disable auto brightness on your phone and dim it to the lowest comfortable level. If you phone supports battery profiles, you can also try to adjust your battery profile to the lowest acceptable level.