Research your topic thoroughly. Find evidence to support your argument and apply it to your speech, but also find evidence that contradicts your side of the argument. Address and refute opposing evidence by undermining its credibility.
Outline your speech carefully. Your audience should be hooked by your introduction, informed by your main address and reminded of your main points by the conclusion. Include space for repetition, as this will help your audience remember your speech.
Write note cards for extemporaneous delivery, which means improvising your speech with the help of notes. Your notes should cover the most essential bits of wording, quotes, strong evidences, statistics and citations.
Practice your speech to yourself over and over again. Stand in front of a mirror and take note of your presence, your gestures and how often you look down at your notes.
Record your speech on tape and note how smooth your delivery is and whether or not your wording sounds pleasant. Eloquence is not everything in speech delivery, but it adds to your credibility and holds the attention of the audience.
Deliver your speech to one or more friends and colleagues who are willing to be honest with you. Encourage them to give you constructive feedback, ask you questions and make suggestions.
Dress authoritatively on the day of your speech. Your clothes, poise and speaking voice should signal to the audience that you are confident and knowledgeable.