Here is a good tongue-twister to try:

How much wood would a woodchuck chuck,
if a woodchuck could chuck wood?

There are 3 keys to pronouncing this well:
1. “wood” and “would” sound the same. (Words that sound the same are called homonyms.)
2. Use the Short “oo” sound to say “wood”, “would”, and “could”.
3. The letter “L” in “would” and “could” is silent.

To pronounce Short “oo”:
1. Lips are rounded (but relaxed).
2. The tongue is in position for a Long “u” (like in the word “blue”) BUT
3. The tongue must be VERY relaxed while holding the Long “u” position.