It's much easier to be convincing if you care about your topic. Figure out what's important to you about your message and speak from the heart.
If you have an important point to make, don't try to be subtle or clever. Use a pile driver. Hit the point once. Then come back and hit it again. Then hit it a third time - a tremendous whack.
A good speech should be like a woman's skirt: long enough to cover the subject and short enough to create interest