Slouching is unbecoming and say say it increases back pain.

I think that's largely outdated thought. Frequently taught and as a result global back pain rises rather then lowers. You get back pain when you freeze your body and prevent it from moving.

If you get an impulse to slouch inside your body follow it and make the movement consciously. That means your back goes back while the shoulders go up and in front.

If you shoulders go down while they are going in front you will likely tense up.

Conscious breathing can also be very valuable as a tool to stay in touch with your body.