I've been using a Kensington cellphone windshield mount for my smartphones for several years with great success.� I mount it low on the windshield and tweak the 12" flexible neck so that it travels over the left dash defroster vent, low on the dash so that it sets between the ash tray and the instrument cowl.� This alignment lets see the display easily, answer a call and put the caller on speakerphone, though I now have a Bluetooth ear piece that I put on when a call comes in.� Additional it can be adjusted as a horizontal mount for GPS or an FM transmitter with plenty of room for power plug to the cigarette lighter. There are multiple vendors of this mount but they all should handle the iPhone sized units or a Nuvi, Garmin but check the specs.
http://www.bhphotovideo.com/c/product/725828-REG/Kensington_K39217US_Windshield_Vent_Car_Mount_for.html

Gary