In this Asana, it is important that you keep the lower back straight. Aside from that, remember that when you are twisting, the hips do not twist along. Otherwise, the right hand side becomes active and the lower back gets "locked up" as it were which results in tension in the lower back. Prevent this by actively tightening the left hand side and let it even help carry the position so that you can keep the hip in its place when twisting. This dynamism in your side also helps you to keep the dynamism in the hip active through the right leg as a result of which the hip is mobilized as much as possible