Staff pose (aka dandasana), is a pose you should add to your practice if you'd like to improve posture. It also strengthens the back muscles and stretches the chest. It doesn't look very challenging, but you'll find that once you're in it, it's more work than you thought. I mean, how often do we sit up straight like that with our legs out? If you've got really tight hamstrings and your legs don't extent straight, try sitting on a rolled up blanket or a couple books. Also, if you find this really difficult, try sitting against a wall until you can sit comfortably away from it. Feel free to ask question questions below in the comments.