The absolute best thing I learned as an undergrad was this:

Place the tip of your index finger on the top teeth at the gumline. Then, have them push their upper lip into the finger, trying to push it down. It will force the corners into the correct position and flatten the chin. Have them hold the corners and then place the mouthpiece with the firm corners and flat chin.

Last year I inherited a move in who played very flat. It took a few weeks, but after reinforcing this daily with her she eventually fixed the issue. I had to continue to reinforce it all the way up to UIL but she eventually figured it out.