I spend almost an hour installing a switch in the robot. To be sure, it is a nice switch, and a pretty decent wiring job.
I found a cool switch with a red plastic throw. There are two sets of contacts on the switch, so I am able to wire both the 9V battery to the VIN of the Arduino and the motor power pack to the switch. So, with one throw of the switch, the battery pack is pumping juice to the motor driver circuit and the Arduino springs to life.
I was careful to provide proper strain relief and to secure the 4 switch wires firmly. The switch wiring has to ride on the hinge of the top box. I bring the positive side from each power source up from the bottom level to one side of the switch, then send it back down to the bottom level from the other side of the switch.