Tip 1: Try to keep everything still. When both the camera and baby are still the area of interest will focus better.
Tip 2: With iPhones, you can use the "focus lock" feature to create a yellow box that will focus on anything within the box. To do this, simply tap once in the middle of the screen and the yellow box should pop up. Try to get the baby's mouth inside this box.
Tip 3: If taking pictures isn't working well then you may record a video and screen shot the part of the video that displays the best image.