I see no one mention sitting on your sitting bones. People have a tendency to collapse their spine at the base, and then rotate their pelvis backwards to slouch, even if their upper body is straight.

If you sit aligned you can feel the bony part of your butt touch the seat, which although uncomfortable at first, makes sitting longer more tolerable after time.

At university we would have a lot of classes without chairs, so we’d sit on the floor a lot, and the above would help.