Here's how I got my Photojojo lenses to stick to my iPhone 4.
1. Bought a Griffin Reveal case to stick the magnet ring to, since it won't stick to the iPhone glass. It has a small enough hole that the magnet won't fall in. The ring centers nicely at the corner of the case.
2. Used wire cutters to chop a segment out of the ring so I can still use my favorite flashlight app. (I rarely use the flash with the camera.)
3. Used needle nose pliers to bend the jagged ends downward into the case so they won't snag on things.