We have all heard about pulling the shoulder-blades together in the back as a quick postural fix. However, that only addresses the top of your posture and not its foundation.
Here is a simple and super-effective posture improvement cue:
First, locate the bottom of your ribcage in the front (the lowest rib you can feel with your fingers).
Second, locate the Anterior Iliac Crest - the pelvic bone in front of your hip (where some hip flexors attach).
Next, notice if the space between these two points (your abdomen) is collapsed/shortened habitually. It often is. We tend to get lazy and drop the ribcage forward.
Finally, elongate this abdominal space. Draw the points of the lowest rib and the anterior pelvic bone away from each other. The ribcage should lift slightly.
This will instantly improve your posture, open the diaphragm for better breathing, and create more space for the internal organs.
In this whimsical sketch, the person A is dropping his ribcage. His shoulders elevate.
The person B is elongating his abdomen, lifting the ribcage, creating more space in the front, and his shoulders naturally relax downward.
On the second thought, you can get a massage, and that will do wonders for your posture, too.