Often enough, I do a lot of leaning and gestures with my characters sitting down. Here's a quick example I made
That red line usually gives an indication of the spine, as well as what kind of pose I'm going for. Course, I rarely draw the red line, but knowing the gesture you want when a character is sitting really helps to show dynamics posing and feelings.
For me, it's all about the emotion for the character. Generally, someone who's defeated, tired, or upset often has a more slumped red line. Someone who's more chipper or excited would leaned in upper, like they're sharing a big secret (like the tiny doodle at the bottom).
These are just a few examples on how I do it, tho, but it helps to know how you're imagining the emotions