The best way to improve your sitting posture longterm is to take it slowly. Start by recognising how you sit and try to come out of the positon you habitually sit in all the time, this is the first step to improve it.
I would recommend sitting on a swiss ball for 10 minutes a day, sit comfortably with your feet and knees far enough apart to feel stable but a little bit wobbly, this way you can tap into your body's natural balance reactions.Have bear feet if you can and make sure your not on a slippy surface , have the ball next to a chair if you're not used to it.
I'll be giving more tips out for you to follow but let's start with this one and see how you get on. Keep me posted!