Sit on a firm, but comfortable seat. Couches are too mushy to support good posture during vocal exercises, so look for something like a stool or a piano bench.
Position your knees above your ankles and your shoulders above your hips. Each part of your body should help support its weight. Avoid leaning forward or backward, as this throws you off balance and ruins your posture.
Hold your head high, but don't push your chin out or into your chest. Think about placing your chin on a table parallel to the ground to find its correct position.
Relax your shoulders and allow them to drop down and back. This not only gives your ribcage room to expand, but it also prevents your jaw and neck muscles from tensing up during the vocal exercises.
Raise your chest so your ribcage tilts backward, opening up your abdomen. This creates space for your diaphragm to move and draw air into the lungs, which is crucial to controlling your breathing.
Let your arms hang loose at your sides. If you're tense, shake them out and then allow them to relax.