Wallee M car mount
Studio Proper uses specialized cases that attach to the Wallee M car mount ($35) with the same neodymium magnets that are used in MRI machines. A heavy-duty suction cup then secures the mount to your windshield. Once it's attached to the car mount, you can adjust your phone for optimal viewability.
There are currently cases available for your iPhone 4, 4S or 5; Galaxy S3; or Galaxy Nexus. Each case costs $30 and is compatible with other Wallee products, like the Pivot Mini for desktops or the Mount disks, which can attach to the wall.