Keep your keyboard and monitor separated by height. Laptops are evil for upper back posture.
Think of how high your eyes are above your elbows. Most of us need the center of our monitors to be 2-3ft above our keyboard height for proper ergonomics. Even typical laptop risers don't cut it.
Get your eyesight checked - and get it rechecked annually, because it can change significantly in as little as six months. Nearsightedness (myopia) and astigmatism may both cause you to lean in closer to your monitor to read clearly, causing your back to hunch. Corrective lenses make this unnecessary (though they alone won't break you of the postural habit.)
Lower back posture is a separate issue - and that's what sit-stand desks, exercise balls, and the like try to address. That said, they also strengthen your core, which may be necessary (though not sufficient) to straighten your upper back.
You may want to consult a physical therapist for specific exercises to straighten your posture, for guidance on how to do them most safely and effectively, and for a program that ramps them up gradually as your strength increases.