what is the correct feeling to have? i'm not supposed to have a droopy look, right?
is the correct feeling to have a slightly sticking out chest, so that it will
look like my back is straight and my shoulders aren't reaching for the ball?
or put another way, if I stood up straight, what is the look and feel that I'm supposed to have with my chest and shoulders?
Have your arms hanging off your shoulder sockets by the weight of your arms. This will give you the "box on the neck" upper back look you want, as opposed to a dead straight upper back.
Another good check to do is from the 'down the line' view (golfer between camera and flag). You should be able to see your left arm. This means your right shoulder is in a nice low position (relative to the left), and your right elbow isn't too high, but relaxed.
Classically, the Left Wrist is Bent, the Right Wrist Flat (Chapter 4), the Clubface and Clubshaft square (putting the Hands at a mid-body location) and the weight evenly distributed (2-J) with the back straight from the neck down--not rounded at the waist. It is easier to control the rotation of a straight rod than a bent one.
Just look to Ernie Els and Michelle Wie for guidance on posture at address.