I was sent a link to to the following great article on power saving techniques for microprocessors and in particular the ATMEga328P which is the chip used in the Arduino.
It is probably the best article I have ever seen on the various ways of squeezing the maximum amount of life out of your batteries and using the least amount of power in your project. Ideal knowledge if you are making any kind of sensor node or other battery operated device and want to make full use of the power saving modes that the Atmel chips offer.
Take a look. It’s a long read and goes into a lot of detail but is well worth the read and definitely one to bookmark: