According to a concept called postural theory, how we physically hold ourselves can influence how we feel. It makes sense, then, that you might want to optimize the way that you sit (you can read more about how your posture affects your mood here).
According to Alexander, sitting in a chair isn't as simple as plopping yourself down: “Make sure that your hips are above the height of your knees when you’re sitting in any position. What that will do is naturally put your lower back and your pelvis into an architectural position of integrity, support, and load-bearing. Drive weight through the shoulders, and [you will] would comfortably stack through that neutral spine.”
In addition to the hips and knees, Alexander emphasizes the importance of getting to know your sitz bones (he calls them “the feet of your butt”), which are located at the base of the pelvis. "Load weight in a mechanically effective way," he explains. "Have a slight tilt forward—not too much to the point where you’re in hyperlordotic territory [aka, when your spine has an inward curve]—and allow the ribs to tuck down towards the pelvis a little bit while you maintain that position.”
Once you’ve become a bit more mindful of your hips, knees, and sitz bones, the next step is to practice some rounds of nasal breathing: “[Place] your hands on the low ribs and just notice the breath [flowing] through the nose," he says. "Just by doing that, you’re getting access to much more air."
Finally, Alexander offers a helpful visualization technique while you sit and breathe: “Imagine there’s a little string in the back of your head, slowly drawing your head up towards the ceiling. Use your visual muscles and look up as opposed to looking down all the time." Looking up, according to Alexander, supercharges the senses. "It's sending the signal that it's time to wake up, time to get creative, time to come up with ideas," he notes.