Sharing this since some of you might find this long read on proper tongue posture useful.
Commonly called "mewing" (named after Dr. Mike Mew), the practice basically involves retraining your brain to have your resting position be mouth closed, teeth lightly touching, and your tongue naturally resting on the roof of your mouth.
By correcting your tongue posture, you can pull your face forward, expand your palate, stop mouth breathing (which has been linked to 'long face,' poor posture, and other negative health effects), and overall have a more healthy-looking poise.
Proper tongue posture is nothing new. It's part of a greater orthodontic strategy known as myofunctional therapy. However, it's surprising how many people don't know what proper tongue posture even is. Many seem to think that resting your tongue on the floor of your mouth is "normal," when it actually causes what's known as "long face" and a recessed profile when coupled with mouth breathing.