Posture, it is the bane of the computer bound employee. And no matter where you work you will see the slackers slouching or resting at comfy angles instead of focusing on improving their posture. And why blame them? This is actually hard to do. And unless you work toward it, the "ideal" posture cannot be forced for very long.
Especially for someone with significant muscular imbalance, stretch and strengthen are the major players here, so never discount that aspect as well.
The "ideal" posture is butt all the way to the back of the chair. If you draw a straight line down the side of the body, your ear, shoulder joint and hip joint should be relatively vertical to each other.
The further forward you are, the more strain you place on your neck, your back, and other joints. Whereas the majority of your joints should be in neutral positions, which are positions that allow the straight flow of blood. If you can't, then go for a 90 degree angle. That is your arm is allowed to flow down from the should, bend, make a 90° bend and rest without stress on your mouse. If you're on a laptop, you'll notice that these rules are actually impossible. So you'll either have to use an external monitor or an external keyboard.
And yes, ideally take breaks as often as feasible. I was told that you should move around and stretch every half hour to forty-five minutes. But when you are busy, in the middle of something, or concentrated, I tend to forget. Still I often shift my position and generally try to keep an upright but soft posture.
It's good to allow yourself some room to sit in a comfortable way without getting too attached to one way, where you may develop a bad habit.