Do not position the violin directly in front of your face while playing or holding the instrument. This is a common natural reaction, because it allows you to look at the strings for finger positioning. Instead, position the violin on your left shoulder, pointing it to your left side. Holding the violin to the left allows for visibility when you are playing, but it keeps the instrument away from the front of your face for presentation.
Do not collapse your wrist directly against the neck of the violin. Violin instructors commonly refer to the collapsed wrist habit as pancake hands. While collapsing your wrist sounds like the more comfortable option, it detracts from the control you have over the violin. You should keep your wrist straight and support the neck of the violin with your thumb and fingers. At first, proper position will feel unnatural and even slightly painful. However, over time the positioning becomes second nature and gives you more power when fingering notes.
Never rest your chin on the surface of the instrument instead of the chin rest. The chin rest is designed to allow you to use the weight of your head to assist your arm in holding up the violin. If you are resting your chin on the actual instrument, reposition yourself and ensure your chin is only on the chin rest. If this is uncomfortable for you to do, purchase a shoulder rest that lifts the violin up for comfort.
Your elbow should never touch the left side of your body, because it would cause you to bend in an unnatural way. Over time, your muscles will strengthen and you will not feel the urge to rest your elbow against your ribcage. Additionally, do not be afraid to drop your left arm away to rest while holding the violin with your chin and shoulder. Dropping your left arm to your side is a sign of an experienced and confident violinist.