I second the recommendation for the Arkon Slim-Grip. This mount works by having one end expandable. So, you put the bottom of your phone in the mount, push down and then let it spring back up and it will snugly hold any iPhone, in most any case (or no case)
The slim grip comes with little feet / holders that reach out to grip your iPhone, the feet are padded, so they wont damage anything. My slim-grip came with 8 feet 4 long ones, 4 short ones. For my iPhone 4s I used all short feet, for my 5 I've switched to 2 short ones, and 2 long ones. You can position the feet anwhere along the slim-grip (there is a grove all along the top and bottom of the slim-grip) This positioning allows you to put the feet where they wont' affect your power button, or block your headphone jack, etc.
One thing I love about the Arkon Slim-Grip is how, when my phone is mounted, I don't see the mount, just the little feet at the top and bottom.
I got mine with a vent clip. It can move a little bit side to side (on the vent), I could fix that if I wanted to with some padding, but it has never bothered me. I love that my vent-clip and slim-grip combination can travel with me, and I've used it in all my rental cars without issue.