arduino uno sleep mode??

i want to be able to put the arduino into sleep mode to save battery power until an action button is pressed. how much power savings can i actually expect and can anyone point me in the direction of how to do it? i have seen many conflicting ideas on how to do this or even if its possible.

I like this because it is simple:
http://code.google.com/p/narcoleptic/

The microcontroller uses almost no current compared to the supporting circuit. Turn off the USB chip and LED. Now you're left with the regulator idle current.

Nick Gammon has written a tutorial on various low power techniques for Arduino class machines: Gammon Forum : Electronics : Microprocessors : Power saving techniques for microprocessors