When you're shooting video with your iPhone, it tends to constantly refocus on images when you move in and out of areas. This causes a lot of blurriness and an overall junky look. To fix this problem, Cult of Mac suggests locking the exposure and focus.
Just launch the Camera app, set it to video, then tap and hold on the screen where you want to set focus and exposure (this is usually the subject you're shooting). Hold that tap until you see "AE/AF Lock" show up on the screen. Obviously you'll now need to stay the same distance and in the same lighting environment for the rest of the shoot, but at least you won't have to deal with the iPhone repeatedly auto-adjusting.
Capture blur-free iPhone vids with this tip [Cult of Mac]