@Ivan, I've been meaning to get back to this, but I forgot. Duh.
Instead of thinking posture for a fighting stance, let's focus on posture in general. Different methods of fighting dictate slightly different forms of stance.
What works well for posture correcting/enhancing is - instead of consciously thinking about keeping your shoulders back, focus on bringing your shoulder blades together. Slightly squeeze those puppies towards each other. Slightly
At the same time, very slightly bring your belly button back towards your spine. Slightly.
At the same time, slightly raise your pecs/heart up. I know that reads weird, but trying it is exactly how you do it. Again, slightly.
Slightly bring your chin back. Like a spider was coming down on a thread from the ceiling in front of your face. Again, slightly.
The reason "slightly" in all of these is important is because it's easier to make them habitual. Kind of fun, too. If you over dramatize the motions they just become a pain in the ash instead of habitual.