I super-glued a neodymium magnet to my GPS mount. There are lots of places on the dash where the steel frame is close enough to the plastic for the magnet to "stick." None directly over the instrument cluster though. It stays in place even during the hardest cornering, but comes loose with just the slightest tug.

Conveniently there is steel right below the glovebox so whenever I park it takes no time at all to pull the GPS loose and stick it upside down below the glovebox hidden away from prying eyes.