Classic tongue twisters are classics for a reason. They work to fix articulation problems. Routine and regular practice is key to mastering each syllable you have difficulty with.  Try these:

She sells sea shells by the sea shore.
She sells sea shells at the sea shore;
At the sea shore she sells sea shells.
She sells sea shells on the sea shell shore.
The sea shells she sells are sea shore shells,
Of that I’m sure.
If neither he sells sea shells, nor she sells sea shells,
Who shall sell sea shells?  Shall sea shells be sold?

I am not the pheasant plucker,
I’m the pheasant plucker’s mate.
I am only plucking pheasants
’cause the pheasant plucker’s running late.
Amidst the mists and coldest frosts,
with stoutest wrists and loudest boasts,
he thrusts his fist against the posts
and still insists he sees the ghosts.

For more take a look at The Ancient Game of Tongue-twisters By Duncan Emrich Chief, Folklore Section, The Library of Congress