It’s your instrument; make sure you know how to use it.
Rehearse in the place where you’ll make your speech if you can; it makes a full dress rehearsal and readies you for the real thing. If you can’t use the venue, use somewhere like it.
Project to people at the back of the room by imagining the breath that you’ll need to make your voice get there and then doing it.
Learn to breathe from your diaphragm for deep, slow, powerful breaths that give you all the oomph you need to project.
Practice hitting the end consonants of the words (‘She sells seashells on the sea shore’ is unintelligible to an audience unless you do).
Use the punctuation (verbal or actual) to pause for breath, which helps your delivery and allows the audience to catch up with what you’re saying. Practice a comma for a short pause (say ‘one thousand’ inside your head) and breath, for a full stop twice that and for a paragraph, three times ‘one thousand’ again.
Rehearse the pauses too because confident use of them will help you to deliver your key points, with real impact.
Emphasise the 2 or 3 key words in a sentence to deliver the real meaning in what you say.
Rehearse practising changes of pace, emphasis, tone and drama until it feels right for you. That’s what rehearsal is for, not simply so you remember what to say, but how you say it too.
If there are words, phrases, or parts of the speech you just can’t say in rehearsal, cut them out or change them because you won’t be able to say them in the real thing.
Speak with your real voice, not your phone voice or your actor’s voice. Using your own voice with its accent, inflection, pitch and tone will deliver the most credible message to your audience.