Good posture is all-important for proper singing. It allows for easy movement of your breath in and out of your body. It also aids in effective projection of your voice and makes the choir as a whole look good.
Stand if you’re able, or sit up nice and straight. Close your eyes and imagine a string running from the pit of your stomach out through the top of your head. Pull the string taut and feel how your body realigns itself for a clean, clear airflow.
Keep your head up, not buried in your book. This is essential for many reasons: letting your voice out clearly and easily, keeping an eye on your conductor and engaging with your audience.
Check the illustration. For proper posture, there should be roughly a 90 degree angle between the centre line of your body and your mouth.