I used an ordinary drama school tuning exercise to retrain my facial weakness which made my lower lip to drop perceptively lower on one side. Vanity was mostly the goal, but when I had learned these exercises, it was for a sluggish pronunciation on some words. I was 18 and enrolled in a drama and mime course in Paris. The simple way to retrain consisted in using words, and reciting the alphabet in an exaggerated style, then slowly arrive to those words that were hard to pronounce and work them to the finish. When satisfied with the pronunciation obtained that way, we would put a marble in the mouth and repeat the exercise, then 2 marbles and more, but the end result was to obtain a perfect elocution of the words or sentence given to work on. Then when perfect in spite of these handicaps, the speed and expression had to be worked on.
Believe me, when you spat these marbles, it was incredible easy to go thru any text, no matter how difficult.
I used this to get my lip back in place and also used certain words that use certain muscles around the lips, while driving, when doing something light like house work or cooking. I am sure most people seeing me move my lips thought I was singing with the radio.
One bonus, though. It acts like a face-lift and give you a nice cat face look, At my age, that is a real bonus!