Interesting tip!
Come to think of it, I always feel like I can stand up so much straighter after a massage.
How I did it: I was trying and trying to just "sit up straighter", but I eventually realized I had let my posture get so bad that I literally couldn't sit up straight. I got a series of deep tissue massages which, although definitely not the most pleasant experience, definitely made a WORLD of difference. I had all sorts of muscles that were shortened or tightened which were keeping me from holding my body normally.
Lessons & tips: If you think massage can help, try to get a series of 3-5 massages instead of just one. If you're messed up enough you can't solve everything in one session, and it also allows you to retrain your muscles between sessions and see if there are problem spots you didn't know about.
Resources: My massage therapist offered a package where you buy four massages and get one free.
Come to think of it, I always feel like I can stand up so much straighter after a massage.