I've written scores of speeches, and coached the speakers on giving them effectively... here's what I know:
1. For a short speech where you know the information by heart, it's usually best to write down what you intend to say and rehearse that a few times -- but then don't use the written speech. Instead, put just the key phrases in order on index cards and use those to remind you of the points you want to talk about, and to help you keep your place as you talk.
2. Don't feel compelled to fill every moment with words. Take your time, speak in paced phrases, and pause after making a point to let the audience absorb that information. A speech is not radio -- "dead air" is okay.
3. Someone said to smile, and that's SO important -- if you look grim and tense, the audience will not be as receptive to what you have to say.
4. This is my favorite tip: Imagine that you aren't speaking to a crowd at all, but to just one person -- someone who doesn't know anything about your subject, someone you're trying to persuade to CARE about it as deeply as you do. Speak to this person warmly, from your heart, just as you would if he was right in front of you.
5. Some coaches recommend eye contact with your audience -- personally, I can't handle that, and neither could many of the people I've written speeches for. Instead, I have them look at foreheads. :-) From the audience, it'll look like you're making eye contact, but you won't be as distracted as if you actually were.
I hope this helps! Good luck!