Well, for starters, you need a couple of simple, on-off electrical switches, rated at 3 amps or more. A couple of single pole/single throw on-off toggle switches should do the job. You should be able to find something suitable at an automotive parts store, a hardware store, or a regular big box store. You will use these switches to turn power on or off on the center rail of each track section.
Next, isolate the center rail copper contacts where the track section plugs into the turnout. Simply use some electrical tape or a similar tape to make sure the center rail copper contacts can't touch each other when the track sections are plugged together, and therefore can't conduct track current. Also, make sure the center rails themselves don't touch each other after the track is snapped together with the turnout. MTH rails sometimes are a tad long, and can contact each other even when they're not supposed to. You can saw and/or file an offending rail shorter if necessary, or simply put some more insulating tape on the ends of the rails to prevent contact with each other.
Mount your on-off toggle switches to a board or panel as desired. Then run a hot wire to one terminal of the switch, and run another wire from the switch to the track section. To hook the wire up to the track you will either need a lock-on for each track, or if you're comfortable with soldering, simply solder the wire to the center rail copper lock-on contact underneath the track section. When everything's done and all track is installed, you can turn your power on or off just like a light switch on the wall.
Should you do it? Of course you should. If nothing else, it would be some good, simple wiring practice, which will serve you well later on down the road. 
