There’s no need to balance a book on your head to check your posture. Bottom line: Your back should have an elongated S shape when viewed from the side. Follow these tips to ensure you’re in alignment.
Neck: Hold your head high and straight without tilting it forward or to the side. Your ears should be in line with the middle of your shoulders.
Shoulders: Pull your shoulder blades back and down to lift your breastbone.
Abdomen: Tuck your abdomen in, but be careful not to tilt your pelvis forward or backward.
Knees: Keep your knees slightly bent and shoulder-width apart.