This can be done different ways. For those of you who don't know the limits of you amplifier and/or whether it can handle varying speaker loads or no loads at all, we highly recommend the ab8ss Audio Switcher. This unit allows for x10 speaker switching.
For a slightly dirtier approach, we switch audio (and video, according to Dave) with a UM506 universal module. First, we combine the grounds of a stereo pair, giving us a three wire system to the remote speakers. Then we simply place the UM506 inline with the ground, allowing us to turn of or on two speakers by cutting one line.
Price of ab8ss: Price of UM506:
(Switches 8 pairs of speakers with 2 inputs) (Switches single pair per unit)
Also requires TW-523 x10 interface @$29.99
Per output channel: $27.37 $18.99
Advantage still goes to the ab8ss as it allows for impedance matching and the second input.