The best way to use the "REM" wire is the way Malkavian has said, but you can hook it up to any 12v source that is off when the ignition is off. For example, the lighter in your car does not function with the ignition off, so you can hook it to that. "REM" stands for "remote", and this allows the amp to turn on and off with the car, or with the stereo if you can hook it up there.
I will be hooking it up to the lighter, and I am also putting a small rocker switch near the gear shift, so I can control whether the amp is on or off myself. Hey, I listen to the radio occasionally, y'know, Howard Stern, AM traffic reports, etc., and I don't need big bass for that!
Good luck with your install.