Power power sleep modes

Has anyone used the low power sleep libraries. I'm trying to put the Arduino into a sleep mode that consumes the least amount of power possible and use an internal timer maybe a watch dog timer to wake it up and run my loop then go back to sleep. This is for a battery powered project and I am trying to get close to 2 weeks out of the battery and every way i can save power would help.

Thanks in advance.

Here is some sample code using Power-Down sleep mode: http://arduino.cc/forum/index.php/topic,78430.msg592490.html#msg592490

Here's a recent experiment using Power-Save sleep mode: http://adventuresinarduinoland.blogspot.com/2012/02/high-tech-night-light.html