Many people mistakenly think that proper posture starts with them driving their shoulders and shoulder blades back trying to pinch them as if they were wearing a posture corrector.
This is wrong because it contributes to lower back being overly bent (aka hyperlordosis). Here is what it looks like:
Operating like this contributes to lower back pain simply because it's so extended. Not good!
So instead of trying to arch your back, a far better approach is to start by tucking your chin!
The cues for this simple exercise are very simple:
As you do it, the neck muscles that are usually overextended start working again and the muscles that needn't be short, elongate.
Next time when you think about sitting with better posture, tuck your chin and your position will improve significantly right away.
I recommend you repeat the chin tucks it a few times throughout the day in order to build up the awareness of your posture and the rest will fall in place.