I like to answer questions on posture on the Quora platform where people ask the general public about anything and everything. This question appeared in my inbox the other day:
Why can't I sit still, and sit properly?
To answer the first question, and you may instinctively recognise this already, you're not supposed to sit still, being upright is a balancing act, an activity in itself, a movement. Admittedly we can do it so skillfully that it looks still to a onlooker, but being upright is about refinding the support being offered by gravity spontaneously every moment. We're an inherently unstable structure, which is fine because the trade off is greater mobility (if you want stability be a tree), but that also means we have to keep "wobbling" slightly because we never manage to find a fixed level of support from gravity. As an aside, gravity may not work the way you think, Isaac Newtons laws on gravity that most are familiar with are also incorrect and have been superseded by Einstein's General Theory of Relativity. Without going into detail, the main thing to know is that the contact you feel of the ground under your feet, or chair under your backside, is not of you going down, but of that surface accelerating up underneath you, and your posture is your ability to "surf" that up thrust. Counter-intuitive, I know!
You're postural reflexes will do the movement to "surf" the ground for you, just as you will be breathed. You don't need to do good posture any more than a fish needs to do buoyancy, it will do you if you don't interfere. Unfortunately, as you've recognised, we do interfere through unhelpful habits that we accumulate through life.
To answer your second question, it may be that you don't have a clear conception of what sitting is in the first place. Interestingly, far more people ask about sitting than standing, yet they're essentially the same thing for the torso. If you look at the diagram you can see that both the sit-bones and the hip joints are below the sacral-iliac joint, that being where the spine (sacrum) joins to the pelvis. So the relationship between your pelvis and spine is unchanged between standing and sitting. You are more stable in sitting with your legs out of the equation, which means there's less mobility to refind the support of gravity. This, in my opinion, is why we find sitting harder than standing and requires a little more skill. So sitting is basically standing on your bottom. The common mistake is rolling the pelvis under and letting the coccyx bare weight. Use the sit-bones as your new "heals" when sitting.
Getting to the chair without compromising your poise and posture on the way is a whole other conversation, but if you've already found yourself a bit slumped on your chair, try "walking" your sit-bones over the chair surface and see if you can find a sense of being "stood" upright. It's easier to refind your poise through movement than trying to attain an idealised shape/posture to hold on to.
Finally, and also for another conversation, is how this is also affected by the natural balance of the head on your spine.
If you're interested in learning more I recommend looking into the Alexander Technique, which I'm naturally biased towards because I'm an Alexander Technique teacher.