Are you doing a reading? Unsure of how you will come over?
Here's an exercise I was taught.
First, find a clean cork, wine bottle type.
Second, print out this poem: (W B Yeats)
Had I the Heaven's
Enrought with golden
And silver threads,
The blue, the dark and the dim cloths
Of night, and light, and the half-light,
I would spread the cloths
Under your feet.
But I, being poor
Have only my dreams;
For you tread on my dreams.
Third, read it through out loud. (You'd best do this while the study door is locked!)
Fourth, insert the cork between your front teeth and hold it there. And recite the poem again, slowly, making sure you enunciate clearly every single syllable, every single consonant, especially.
Do it again, until you are certain you are actually sounding every single sound there is.
Now take the cork out, make a few silly faces to stretch the jaw muscles etc.
And recite it again.
Can you hear the difference? Magic isn't it!
(On the day, don't sit there in the venue practicing with a cork between your teeth, or you'll get locked up. But you can do the jaw stretches in the loo...!)