I thought everyone knew this, but since I regularly encounter people who don't, here it is:
If you're having trouble getting the focus right on your pictures, or if you're trying to take the picture at a precise moment, touch and hold the "take-a-picture" button. The camera will focus on what's in the center. When you're ready to take the picture, just release the button.
If you change your mind and don't want to take that picture, drag your finger away from the button, then release.