I just had a weird idea. I've seen a fair few variations on the theme of counting from 1 to 10, counting up or down. But what if, as the count from 1 to 10 progresses, your age is regressing. You get more and more excited as the numbers rise, and that just makes the suggestions more powerful. By the time you reach 10, you're so happy, and bouncing - incredibly proud of yourself, because you're at just the right age; you just counted to 10, and you're young enough that getting that right seems a big achievement. After listening to this file, you'd be so proud to tell anyone who'll listen that you counted to 10 on your own (and you'll presumably have other childlike attributes appropriate to that stage of development)
Hehe ... to be evil, maybe you have to count up to your real age before you can return to normal. You'll need someone to help you, as even counting to 10 is a challenge, effectively putting another person in charge of when you can revert.