Learn how to correctly articulate each of the individual standard American English phonemes. There are 44 phonemes in the standard American English dialect. Spend a few minutes each day practicing the lip, tongue, teeth, palate, gum, ridge and jaw placement required to properly articulate each phoneme until you become proficient at pronouncing them all correctly. If you find yourself having difficulty articulating any of the phonemes, try standing in a mirror as you practice your articulation of the phoneme so that you can observe and correct your lip, tongue, teeth, palate, gum, ridge and jaw placement.
Apply what you've learned about articulating standard American English language phonemes by blending them together to articulate words. A good way to hone your articulation skills is to practice English tongue twisters. Tongue twisters are phrases or passages that are, on average, difficult to articulate. The goal is to maintain crisp articulation as you endeavor to voice these tricky turns of phrases. For example, repeat each of the phrases "six thick thistle sticks," "a big black bug bit a big black bear, made the big black bear bleed blood," "friendly Frank flips fine flapjacks" and "red lorry, yellow lorry, red lorry, yellow lorry..." several times.
Practice reading English poetry passages aloud. Beyond the mere crisp articulation of individual words, there's a subtle rhythm that emerges in spoken language when words are strung together to communicate an idea. Practicing reading poetry aloud is a great way to hone your spoken word rhythmic skills because there's an inherent overt rhythm to poetic language.
For example, work on sharpening your vocal delivery of the this classic English poem written by Robert Frost:
Whose woods these are I think I know.
His house is in the village, though;
He will not see me stopping here
To watch his woods fill up with snow.
My little horse must think it queer
To stop without a farmhouse near
Between the woods and frozen lake
The darkest evening of the year.
He gives his harness bells a shake
To ask if there's some mistake.
The only other sound's the sweep
Of easy wind and downy flake.
The woods are lovely, dark and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.