It's simple. Just put them in a small pile on a table near your piano. (Don't put them right on the piano since they might fall in.) After you've done that, pick a short musical passage to practice. It can be written music, a difficult chord progression or anything else you like, but keep it relatively short.
Now simply play the passage once, and move one of the coins a few inches over on the table, to start a new pile. Then play the passage again and move another coin to the new pile. Repeat until all the coins are in the second pile. Congratulations, you've now practiced the passage 50 times!
This technique might not seem like much, but it's proven to be extremely effective among my piano students. I use it myself too. For one thing, it eliminates "willpower" from the equation. It stops being a question of "Do I really want to play this again?" and becomes a physical thing: "I'll just keep playing this until the pile moves."
You sit down at the piano, push coins on a table and before you know it, you've practiced something 50 times. Have fun!
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