I used to run a workshop on public speaking. There's a lot you can do to make yourself more persuasive, and there are a number of books on the subject.
In addition to hitssquad's advice of telling them what you're going to say, saying it, then telling them what you said, here are a few more tips:
Have no more than three main points. At the beginning of the speech, outline those three points for the audience, make sure you can connect each of the three points and do that for your audience as well.
In addition to choosing your words carefully and rehearsing the text so it flows smoothly, also rehearse your diction and timing. What do you want to emphasize? Make sure you say it in a way that emphasizes it (inflection and intonation...you don't want to bore your audience with a monotone speech). Choose words that fit your own speaking style, otherwise you'll stumble when you try to change your style to fit the speech. Practice clear diction...say every consonant...so your audience can understand you; we often don't notice how many words we drop letters out of when speaking. Do you use filler words when you get nervous or can't think of the next word? (Ah, um, like) To some extent, practicing your speech so you know which word comes next will help with that. In addition, have a friend help you out by practicing speaking in front of them, or answering random questions, and have your friend point out whenever you say a filler word so you get used to hearing yourself say it and can stop doing it (we usually don't notice our overuse of these fillers until someone else points it out to us).
Also, consider your mannerisms. Standing stock still is just as bad as pacing relentlessly. If you are using visual aids, such as slides, remember to face the audience, not the screen. Try not to fiddle with pointers, microphone cords, or any other distractions (get to the room ahead of time and remove anything you suspect will be a distraction if you can...such as an extra chair you might be tempted to lean on). Watch your hand gestures. If you're someone who talks with your hands, control that urge. Excessive gestures can be distracting. If you need to gesture for emphasis, keep your elbows down and outward and your hands up and centered...this keeps attention on you and your face, not out at your fingertips somewhere else.
Do you need to use a microphone? Find out ahead and practice according to the type of microphone you will have. If you will have one of those small ones that clips onto your clothing, attach it near your collar on the side of your body that is in the direction of the screen or center of the stage if you are not standing directly in the center facing the audience, this way you are talking in the direction of the microphone, not away from it, so it will pick up your voice more clearly. If you are going to be standing at a podium or using another form of stationary microphone, adjust it so it is directly aimed at your mouth and speak into it from about 6 inches away...this is the optimal working distance for microphones...any closer and you will sound garbled like you've swallowed the microphone...any further and the microphone won't pick up your voice as well so you'll fade out. Also, with that sort of microphone, remember if you turn your head to look at something else, such as a screen, then you need to do so in a way that keeps your mouth near the microphone or else nobody will hear you when you turn your head.
Some of this is obviously presentation style, not just how to write a persuasive speech, but it all goes together for giving an effective presentation. Plus, when you know about things like hand gestures and how the AV equipment works, I find it helps reduce the nervousness because you know if there's an equipment failure, you can handle it. Practice for those eventualities too...what happens if the projector dies and you have to talk without slides? Perhaps have your figures printed and in a folder so you can take them out and re-create the main points on a whiteboard (but keep that tucked away if you don't have problems so you aren't tempted to fiddle with the paper while speaking). Or, be prepared to speak extra loudly if the microphone doesn't work. Those are really the two main AV issues to worry about, and being prepared and at least *appearing* unruffled will make you look even more professional should there be a problem.
Even seasoned professionals get a little nervous before public speaking. I need to get to work now, but will try to return later and give you some tips on how to hide that (you can't just tell your hands to stop shaking, but there are things you can do to keep the audience from noticing).
By the way, does anyone think this thread should be moved to a more general topic? I think effective speech-making is a topic everyone needs to know how to do well, so while it does relate to social sciences, it might be more useful to be put out there where more people will see it?