Choose an argument about which you feel strongly. If you have true passion for the topic, you will be more convincing. For example, if you are debating the death penalty, do not argue for it if you are actually against it. The audience may be able to detect insincerity from your tone of voice and facial expressions.
Research ahead of time. If you know the material well, you will have more confidence. Backing your argument with solid references makes you appear more credible to the audience. For example, if you want to cite a statistic that is not common knowledge, tell the audience where you retrieved the information. Learn about your opposition's argument to be prepared for any unexpected attacks.
Show respect to your opponent. Do not be rude to your opponent just because he has a different viewpoint. Refrain from calling your opponent names or talking down to him. If you want him to treat you with respect, you must do likewise.
Refrain from interrupting an opponent who is speaking. No matter how eager you are to get your point across, wait until your opponent finishes before you speak.
Speak slowly and pronounce your words clearly. No matter how nervous or rushed you feel, ensure that your audience is able to understand what you are saying. Think about your strongest arguments and state them early in the debate.