Practice a lot when you are not too tired; exercising those muscles and building a habit of standing/sitting up straight will slowly increase your ability to do so more consistently.

I am much better able to maintain good posture comfortably and for longer periods of times when standing, and setting up my home computer with a standing desk has improved my posture and reduced my back pain.

I have also heard it recommended to have a mirror in a place where you will see yourself slumping; it will motivate you to sit/stand up when you see yourself. This has not been very practical for me, but may work for others.

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I think that's largely outdated thought.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.