A phrase designed to be difficult to articulate properly is widely used to improve accent and pronunciation.
She sells seashells by the seashore.
Betty Botter bought some butter, but she said the butter’s bitter. If I put it in my batter, it will make my batter bitter, but a bit of better butter will make my batter better.
Fuzzy Wuzzy was a bear. Fuzzy Wuzzy had no hair. Fuzzy Wuzzy wasn’t very fuzzy, was he?
How can a clam cram in a clean cream can?
Six slippery snails slid slowly seaward.
Pad kid poured curd pulled cod