So you want to use your phone to get directions and other updates in your car? You are going to need a decent car mount. These rear view mounts can help:
Universal Rear View Mirror Mount: this is compatible with the iPhone, iPod Touch, GPS devices, and other phones. It does not block your mirror and gets the job done.
Rear View Mirror Bendy Mount for Mobile Phone: clips to the top and bottom of your rear view mirror. The cradle expands from 1.57 – 3.62″. It supports landscape or portrait positions.
RAM Rear-View Mirror Mount Holder: designed for iPhone 4 and 4S. It has a decent and rugged construction. It will accommodate rails from 0.5″ to 1″ in diameter.
These mounts let you attach your smartphone to your rear view mirror. They are not super expensive which helps.