You may want to try this thing that I bought off of eBay. You get a 1.5" dia flat magnetic piece to put inside the back of your phone case. Then the main part slides into/onto your A/C vent louvers on the dash. Then your phone just stays on it magnetically. Works well and it is pretty cheap....
StalionŽStand Universal Cell Phone GPS Air Vent Magnetic Car Mount Cradle Holder | eBay