Sitting up straight and not slouching helps. Also when you stand put your shoulders back,chin up and your back straight. Exercise helps too.
Probably because your back is in the sitting position, and it gets stiff, then when you stand up, your back is transitioning into another position.
Well you should have sit up straight when you were younger so you are used to sitting up straight. But if that's not it then you could tape a ruler to your back. What I do is make my car seat vertical so my back stays straight the whole ride and I get more and more used to sitting straight.
The muscles in the back, particularly the erector spinae muscles, are primarily responsible for helping you maintain an upright posture and sit up straight. Good core strength and engagement of abdominal muscles also play a role in supporting proper sitting posture.
One example of good posture is to stand with your shoulders back, knees straight and chest out. Another good posture example while sitting is to sit with your back straight and your shoulder back. Keep your feet flat on the floor and avoid sitting in the same position after 30 minutes.
Sitting up straight
both feet on the ground forward in the chair with back straight
No. Do what your mother said, and stand up straight!
stand up straight with shoulders held back
The sitting position involves supporting the body's weight on the buttocks with the back straight and feet flat on the ground. It is a common posture for activities such as working at a desk, eating a meal, or watching TV. Sitting for prolonged periods can lead to health issues, so it's important to take breaks and maintain good posture.
http://www.medicalnewstoday.com/articles/117470.php
The upright position, at 90 degrees, causes disks to move the most, while the relaxed position (135 degrees) causes disks to move the least.