Most electronic gauges instruct you to wire them to a switched power source like an accessory terminal in the fuse box. This is usually the best way to wire them since you will not have to worry about leaving them on after shutting off your car. If for whatever reason you want to be able to shut them off while the ignition is on simply put a toggle switch on the power wires but still run them to a swithed power supply in the fuse box.
switch on, ignition on = gauges on
switch off, ignition on = gauges off
switch on, ignition off = gauge off
Hope this helps.