It also helps to sit on a good chair, which isn’t necessarily the most comfortable. I currently sit on a used front seat from some ~1995 Subaru, and it used to be horribly uncomfortable but it forced me so hard to face forward and upright that I eventually got used to, which likely is not too far from a correct posture. That implies how wrong I am to be on this prestigious forum - but it helped my posture far more than a comfortable chair that faithfully held my body at a “sack of potato” posture.