Honestly getting too close to the screen can making focusing harder. More distance from the screen is less you have to move your eyes around! Thankfully I like wearing glasses so my terrible eyes aren't that much of a burden to me, heh.
Also, consider having a light on in the room, lamp or ceiling fan or whatever. A lot of people tend to play in the dark, generally speaking it's somewhat damaging to your eyes as well. I know that's not posture but you know, relevance.
Anyways a good tip to break the habit is just to catch yourself doing it and consciously pull back and remind yourself. Just casually leaning back into your chair isn't going to help break the habit, usually.