Stand up as straight as you can, with your back vertical and your arms at your sides, relaxed. Then, have a friend take a strip of duct tape and run a three inch strip down your back. The top of it should be on the center of one of your shoulder blades and end three inches below it. Then, that person should put a second strip, starting at the center of your other shoulder blade and going straight down for three inches. Take a third strip and apply it horizontally, connecting the tops of the strips, then a fourth strip connecting the bottoms of the vertical strips. You should have a rectangle on your back, nearly square in shape.
Here’s the thing. As you go through your day, every time you attempt to slouch, that tape will tug at your skin, resisting a poor change in posture. It’s not painful (unless you have excessive hair back there), but it is enough of a physical reminder to cause you to naturally keep a good posture.