Battery life is often overlooked when building projects, especially for beginners. This tutorial takes you through the setup of power saving modes for the Arduino. Utilizing the watchdog and sleep functions, they put the chip into a hibernation mode between cycles. An optimum configuration could take your battery life from 4 days to about 3 years. For a lot of you, this is old news. But for the rest, this is really good stuff. You can download a sample application from the site that mimics the singing of a nightingale when the sun goes down.