Are you sitting at your computer with a phone, iPad, iPod, tablet, or notepad?
Or are you seated on a couch with a printed something-or-other as the TV is blaring news, gossip and ads? Are you seated at the kitchen table, dining table with something to eat?
Even if you're out in the world, visiting, conferencing, waiting for doc, lawyer, accountant, or friend -- you're relaxed, kinda taking it easy with furniture supporting you.
Probably your back is rounded, and your head is thrust forward -- yes, you're an S.
In fact, everyday, most of the day you are an S, not an up and down vertical body.
If you don't fight it, you'll lose it.
Be tall every time you get up to get anything, every time you head for the bathroom, or go into another room -- chin high, fold your hands behind your back as if you're pacing. And smile proudly, inwardly -- be jolly, perky, peppy like ... A lollypop, and keep singing the song as you sit back down.
And try a bit of exercise. It's very easy.