Red Lorry, Yellow Lorry.
Peter Piper picked a peck of pickled peppers.
Tongue twisters and nursery rhymes make such wonderful vocal exercises to loosen your stiff tongue. They have long been used by actors, politicians, and speakers to help them speak more clearly because they're just that effective.
In a vocal training class I attended, we'd recite several tongue twisters in order to feel the "r'" and "l" sounds roll out of our mouth. Saying your vowels, like eee, aaa, uuu, and ooo, aloud continuously can help make your voice come out strong and pure.