What is a winner, you may ask perhaps? It depends on your comfort level with the golden subject. That will change the tone and emphasis of your presentation, and this communicates itself to your audience in many ways.
Excessive pauses, 'um'-ing and 'ah'-ing, and apologizing or appearing apologetic for one's position are common early mistakes, and can be easily handled by picking the right one: a winner speech topic so to speak :-)
Assuming you get stuck with defending speech topics you disagree with strongly, try imagining a debate against yourself speaking for both sides, and notice the flaws in your argument as your imaginary debate progresses.
Then, criticize your mistakes as strongly as possible without being rude or abrasive. If you are not convinced of the rightness of your side after this exercise, repeat it, only this time, imagine that your alter ego is talking complete nonsense.
May I suggest you read Jim's chapter on perssuasive subjects.