I wrote an article about proper posture when sitting in a chair. The amount of comments on it claiming that slouching was actually better kind of boggled my mind. I even tried slouching all day...
for science! My back hurt the next day. If I'm writing and I notice that I am slouching, I put my back against the chair and slide it forward until I can type without compromising my posture. This usually happens 500 times a day because my chair always rolls back
If you slouch, consciously make yourself sit properly for a day, especially if you're on a computer, it makes a huge difference... For me at least. If you are using a shitty chair, buy a good one. I used to use a chair that I swiped from my kitchen. Worst idea ever. A huge pile of money later, I have a chair that's as pleasurable to sit in as a blowjob is to receive.
Blowjob chair.