A "drill" I can recommend is writing out words a defined number of times (pick a number you like between 10 and 100), as you write also pronounce them without letting your tongue lose contact with the roof of your mouth. This may feel very unnatural at first. If you don't like the result just recycle the paper.
There's also the habit of sucking or pulling in your tongue - don't do that. I think it causes jaw clenching. If you want a fast way to get rid of that, invent metaphors for it and then watch yourself do it automatically in social interactions.
Also check out ortothropics, it's the emerging science of facial development through oral posture. They have a large collection of instructional material on YouTube.
Haven’t tried either thing but I hope to soon. In my experience dentists and orthodontists are little help with these kinds of problems.
1) The establishment/western orthodontics school where teeth are the root of the problem so are moved around or removed in order to achieve flatness in the face profile. Maintaining results might require wearing a retainer for life.
2) The holistic orthotropic approach where misaligned teeth are a symptom of other issues. It emphasizes myofunctional exercises (for tongue and cheek posture), osteopathic adjustment to realign skull sutures, neck posture and breathing exercises to train the body to regulate its CO2 during sleep. The results are permanent since the patient's efforts help drive bone growth in the palate and changes in the jaw joint.
I sucked my thumb until I was 9 which left me with a severe overbite, then had braces and headgear for 8 years as a child (luckily had no teeth removed) but it left my mouth too small. I am a late bloomer, probably 10 years behind my peers. I began weightlifting in my early 20s and basically have a different body type now than I did in adolescence, so my jaw grew and constricted my throat, which caused sleep apnea for at least 10 years. I've been chronically exhausted most of my adult life and didn't know why. That led to poor self image/self esteem and chronic melancholy/depression although I have never been diagnosed or taken medication for it.
I got fitted with an A.L.F. appliance about 9 months ago and the results have been nothing short of miraculous. I'd say my throat has opened by 1/4 inch or more, my skin has cleared up, my eyebrows have grown back, even my digestive issues have improved dramatically. And to top it off, I just look healthier/happier and feel more attractive in public because I used to have a gummy smile with no teeth visible at the sides and a recessed chin (which are the main causes of babyface).
For anyone reading this, I'd be very wary of orthodontic treatment that requires extractions or headgear. Get a second opinion, and opt for a Herbst device, A.L.F. or D.N.A. appliance instead. Invisiline is fine. Also look on youtube for exercises to improve your jawline, such as chin and tongue frenulum stretches. The most important exercise I've found is to keep your tongue suctioned to the roof of your mouth and learn to swallow with it up like that. The constant pressure of the tip of your tongue above your front teeth causes the nerves to stimulate bone growth to provide room for your top teeth. Then the bottom jaw conforms to the top, which can cause the bottom teeth to spread and straighten as well. But really, find a myofunctional therapist because this is only the tip of the iceberg. You can usually train with them remotely.
My theory for why this is not mainstream knowledge is that there is not as much money in a simple wire and holistic treatment/exercises as there is in orthodontics.