Keep the subject in focus. It's easy to accidentally focus on your hand instead of the device you're working on.

For most smartphones, you have to tap the area on your screen where you want it to focus.

Use a self-timer. Believe it or not, just having your shaky hands on the phone can introduce a little blurriness into the shot. Set your camera to a short delay using the self-timer setting so that you can remove your hands completely after pressing the shutter button.