Hold your core ! Also keep thinking about having equal weight in both sturips. Hope this helps
Hold your core, it is painful to hold your core and slump at the same time. Hope this helps
Try to be actively thinking “shoulders back” and fix it. Easier said than done though! There are also devices (idk what else to call it) that pull your shoulders back and help your posture.
It’s just a habit. You can make a habit out of anything, including keeping your perfect. The quicker you stop the bad habit, the easier a better one kicks in.
don't grip with your legs, let them hang loose and you will sit deeper and taller and straighter
When I used to have this issue I used a riding aid to help with my shoulders there is a device on PS of Sweden’s website called ‘perfect shoulder’ that is similar to the aid I used!
Hope this helps