My son was just struggling mightily with one of the classics. So, let us share our favorites here, and see how we all do. My offerings:
**She sells seashells down by the seashore.
How much wood would a woodchuck chuck if a woodchuck could chuck wood?
Lazy lizards love laying on logs.
Skip snapped shots of shiny skies.**
Your turn. 
Cartooniverse
If you could say The Sixth Sheiks sixth sick Sheep three times fast I’d be amazed.
knapsack strap.
or The sixth sick Sheik’s sixth sheep is sick.
Which wristwatch is a Swiss wristwatch?
Bake big batches of brown blueberry bread.
There was a young man called Fisher
who was fishing for fish in a fissure.
Then a cod with a grin
pulled the fisherman in.
Now they’re searching the fissure for Fisher.
I bought a bit of baking powder and baked a batch of biscuits. I brought a big basket of biscuits back to the bakery and baked a basket of big biscuits. Then I took the big basket of biscuits and the basket of big biscuits and mixed the big biscuits with the basket of biscuits that was next to the big basket and put a bunch of biscuits from the basket into a biscuit mixer and brought the basket of biscuits and the box of mixed biscuits and the biscuit mixer to the bakery and opened a tin of sardines.
real rear wheel
How about Tom Lehrer’s wonderful pastiche of “Clementine” as if written by Gilbert and Sullivan:
Very very fast!
Twixt six thick thumbs, stick six thick sticks. I can’t even read this one without getting confused.
Six thick thistle sticks.
Three smart fellows, they felt smart.
I slit a sheet.
A sheet I slit.
Upon a slitted sheet I sit.
that was hard even just to type!
My kids get a kick out of these:
**Shut up the shutters and sit in the shop.
I’m not a fig plucker, I’m a fig plucker’s son.
But I’m gonna pluck figs until the fig plucking’s done.**
I wonder why my sisters won’t let me sit at the kids’ table anymore?
A couple of simple ones:
**Red leather, yellow leather.
Lemon linament.**
Oh and here’s a great theater warmup.
I am the mother pheasant plucker, and I pluck the mother pheasants. I will pluck the mother pheasants until the mother pheasant plucking’s done.
I am not a mother pheasant plucker, I am the mother pheasant plucker’s son. (and so on…)
We have a family one discovered by accident years ago:
Which rich wheat rancher?
Another theater warmup: The big black bug bit the big black bear to make the big black bear bleed blood.
Couple of shorties:
Toy Boat (say it 5 times fast)
Sunshine City (at least 3 times fast…)
**I am the pheasant plucker. I pluck mother pheasants. I am the most pleasant mother pheasant plucker that ever plucked a pheasant.
Cinnamon M&M Enema.
You know New York. You need New York. You know you need unique New York.
Sven said, “Ten tents,”
Ted sent ten cents.
Sven said, “Ten tents,”
Ted sent ten cents.
Sven said, “Ted, send ten tents,”
Ted said, “Sven, send ten cents.”
When Sven sent Ted ten cents,
Then Ted sent Sven ten tents.
Betty bought a bit of butter
But the butter was bitter
So Betty bought better bit of butter
To make the bitter butter better
Try saying “Unique New York” fast. I can’t even say it once fast without saying it like “you-neek-you-nork”!
Boy do I miss Danny Kaye! Now there was a guy who could do tongue twisters! The best part was they were always in character, and relevant to the plot of whatever film he was in.
My favorite was in “The Court Jester” during his exchange with the King. And he did it rapid fire:
*
King Roderick: Ah, I must admit, I was shocked to hear about the Duchess. What did the Duke do?
Hawkins: Pardon?
King Roderick: The Duke. What did the Duke do?
Hawkins: Eh… the Duke do?
King Roderick: Yes. And what about the Douge?
Hawkins: Oh, the Douge!
King Roderick: Eh. Well what did the Douge do?
Hawkins: The Douge do?
King Roderick: Yes, the Douge do.
Hawkins: Well, uh, the Douge did what the Douge does. Eh, uh, when the Douge does his duty to the Duke, that is.
King Roderick: What? What’s that?
Hawkins: Oh, it’s very simple, sire. When the Douge did his duty and the Duke didn’t, that’s when the Duchess did the dirk to the Duke with the Douge.
King Roderick: Who did what to what?
Hawkins: Oh, they all did, sire. There they were in the dark; the Duke with his dagger, the Douge with his dart, Duchess with her dirk.
King Roderick: Duchess with her dirk?
Hawkins: Yes! The Duchess dove at the Duke just when the Duke dove at the Douge. Now the Duke ducked, the Douge dodged, and the Duchess didn’t. So the Duke got the Duchess, the Duchess got the Douge, and the Douge got the Duke! *
It’s always this one that gets me:
Red lolly, yellow lorry
(Ten times aloud and fast)