Ali, an eight-year-old piano student, was having trouble keeping her wrists level. She kept resting them on the piano case while she was playing, thus creating tension and restricting motion. Every few seconds, I had to either verbally correct her or gently nudge her wrists up.
Because I secretly desire to be a mean piano teacher rapping knuckles with a wooden ruler, I did this:
The tape is sticky-side up, fastened with a piece of folded-up tape on either side.
This turned out to be an immediate, complete solution to Ali's problem. She never touched the tape, and I never had to remind her not to. She simply decided that she didn't want to get stuck to the tape, and reminded herself to keep her wrists up.
Proof that to break a habit, everything you really need is in your own mind.