Additional minor tips on the swag/body language in condensed form...

English has this incredibly stupid phrase "suck in your gut." This gives people the misleading impression that they need to draw up their diapraghm, and they end up effectively holding their breath. This is, obviously, kind of hard to maintain. What one actually needs to do is not "suck" but "pull" one's gut in. This relies on the rectus and tranversus abdominus muscles and should have no effect on one's ability to breath when done properly. This can be rather hard to master when one is not already in shape. My suggestion is to work on pulling your spinal column straight from both the back (erector spinae) and front (abdominus) muscles simultaneously. Imagine a force simultaneously pulling your tailbone straight down, and the top of your head straight up, and tense your middle to maintain that elongation.

Eye contact is hard. Fortunately, from more than a couple feet away, no one can tell that you're staring at their forehead or the bridge of their nose rather than into their eyes. Looking at the forehead has the additional advantage of forcing one's chin higher; women find men more attractive when looking at the male face from below. Even if you're not making eye contact, when just walking down the street, it's good to practice looking straight forward or even slightly upward, not at one's feet as most introverts are inclined to do. This is easier if one is already following the advice from the previous paragraph.

To get the taut, puffed-out chest effect requires using not the pectoral muscles but the back, by drawing the shoulder blades slightly together. This can be made more natural by practicing standing/walking with the palms of the hands facing forward.