Get in focus with AE/AF lock
The auto focus on the iPhone is a fickle mistress. If you're having trouble with auto focus jumping around, you can touch the screen until you see AE/AF lock in yellow letters (auto exposure/auto focus lock) and lock it in place.
Know your shutter
The touch-screen shutter button isn’t nearly as user-friendly as an actual camera’s shutter. For one thing, make sure you account for any glitchy software lag time, and always hold your phone in the same position a few seconds after taking your shot. Also, keep in mind that the photo doesn’t register when you press the shutter, and only captures once you release it.