Your head weighs 10 to 12 pounds and when you're slouched its weight on the cervical spine is even heavier. This can lead to neck and back pain. Habits of slouching can be hard to overcome if you try to "muscle" your way to good posture. The Alexander Technique can help you learn to use your smartphone and other devices without slouching.
Here are a few tips to get you started:
Bring your smartphone closer to eye level so that you can keep your body more erect.
Think about the space between you and your device.
Let your eyes lead the movement to look down at your smartphone (moving from where the skull balances on the spine which is higher than you may think).