I always just modify the main loop so that it reads
Code:
if(autonomous || p4_sw_trig)
{
... calls into autonomous...
}
Plug in the port 4 joystick, hold trigger, and then your robot starts doing its autonomous routine. Let go, and it returns to teleop mode.
Then I mark off the port 4 joystick port on the OI so that no-one tries to use it in the competition. This way, you can immediately stop execution of autonomous by releasing the button if it starts doing something dangerous to humans or itself, you don't have to remember to copy code around, you don't risk frying your OI, and you don't need a special piece of hardware.