Go the Apple store, or order online a "Crystal" film for the iPhone. They make 2 versions, a smooth and an anti-glare. Be sure to buy the correct one that you want.
I use the smooth, and it is virtually invisible, its smooth, no degradation of the image quality. It works perfectly
I would suggest buying a couple of them because applying them can be tricky due to dust, finger prints etc. Its not too hard to do, but you can sometimes get dust trapped underneath the film when installing it because it takes a bit of trial and error to line it up nicely.
Its the best "skin" for the iphone. Stay away from the invisible shield like products. They dont work well on the iphone and they peel off because the corners do not really adhere well. I had one on the back of my phone, and used a crystal clear smooth film on the front. Eventually the corders of the invisible shield peeled up, and dust got underneath and it looked like poop. So i took it off. BTW when you take those Invisible Shield like products off, they peel up the ink on the back of the phone used to print your serial number. Luckly it didnt take it completely off, but i've heard of it happening. So be adviced.
Anyways, in the end... i realized i didnt need the Invisible Shield skin. It just doesnt do much for the phone. The Invisible Shield worked great on my iPod, but it just doesnt work on the iphone cause its edges are too rounded.