Assuming you use a wall wart power supply, you can add a toggle switch inline with it:
WW +----- --------o o---- Ard+
WW - --------------o o---- Ard -
The \ is the switch. Keep in mind a wall wart will use some power even if the switch is open.
An alternative method is to use an X-10 module (or other AC powered switch) on the wall wart to switch it on and off.
-transfinite