Tongue Twisters are a great way to work on one's enunciation because they are so challenging to do. When we work on something hard, the easier stuff because so much easier!
First say them slowly. Second, increase the speed. Do so in an iterative manner increasing the speed until you can't keep up:
She sells seashells by the seashore.
The thirty three thieves thought that they thrilled the throne throughout Thursday.
Peter Piper picked a peck of pickled peppers.
A peck of pickled peppers did Peter Piper the pickled pepper picker pick.
If Peter Piper the pickled pepper picker picked a peck of pickled peppers,
Where is the peck of pickled peppers that Peter Piper the pickled pepper picker picked?