The moment you start to put yourself into a posture or position you start to fix. Your forcing yourself to be there. You're making everything worse. You put one layer of bad habits on top of the ones you've already got. So you're fighting yourself. Eventually you'll give up. Some people might be able to do it for a really long time, others might not be able to do it for very long at all. At some point your body will cave in. And you'll be creating a lot of harm to yourself because when you put your body in a position and hold it, you fix your muscles and that is going to create shortening and tension all over your body.
If you think that you just hold your shoulders in a certain position and that's all it's going to affect, it's not the case. It'll affect your neck, your head, it will affect things in your hips and legs as well. It's going to have a knock on effect all over the body.
So that's what you don't want to do. You don't want to hold your body in a position and fix it into place.
If you want something to do to improve your posture then very simply go for a walk and don't look at the floor. Because if you look at the floor you're going to cause lots of harm to yourself by pulling yourself down. It doesn't mean you can't look at the floor ever, you just have to let your eyes lead the way instead of shortening the neck and pulling the head forward and down. You don't want to be staring at the floor the whole time. Creating that shortening down the front and curving the back into that C shape.
Or do semi supine for 20 mins.
If you look at babies or animals they're not trying to hold themselves up it's easy for them. And they have fantastic posture. Cultures where people aren't conditioned to sit in chairs, like eastern or african cultures they have much better use or posture. They use their backs more efficiently and haven't been sitting in backwards sloping chairs. They're also able to sit in squats in a way that strengthens their back naturally without having to do any kind of work out. They can also walk for miles across the desert with pots of water on their heads. Something that would endanger a bodybuilder. So there is a different strength that is available in the body, but we have to learn how to cultivate that.