Need help with on off switch to power arduino. I am trying to add an on and off power switch to my arduino project.  My project is running off a 9v battery and I would like to save as much power as I can.  There might be a better why that I don’t know.  I added SLEEP_MODE_PWR_DOWN to my project and from what I can tell, the arduino enters sleep mode.  I am having a hard time getting the arduino to wake from sleep mode.  Am I able to do this from a full power down?  I have been reading this link. http://www.gammon.com.au/forum/?id=11497  and others.
Since you are using an Arduino Uno board, putting the MCU (the ATmega328P-PU) to sleep won't save you much battery juice. Putting the MCU to sleep won't make a difference because the voltage regulator on the Uno will waste a lot more power than anything else on the board. The ON LED will also draw significant amount of current on that board.

To really save power you should look into getting an Arduino board that has a more efficient voltage regulator, or use no regulator at all by changing your 9V battery by, say, 4 NiMH AA cells.

Here's some more information about the issue for you to look at: What are (or how do I use) the power saving options of the Arduino to extend battery life?