On & Off Switch

I’m almost done with my LM35 temperature sensor project and need to make an on & off switch. What is the best way to do this?
Do I put the switch on the positive side or the negative side?
Also is there a way to turn the entire Arduino on & off if using a wall plug that will always be plugged in but not easily accessible?

I think sv1 will switch the power from the plug to the serial/usb port. You could replace this with a switch.