Some people think 'being a confident speaker' is the most important skill in debating. It isn't. Quality of arguments is what will win a debate. That said, how you speak is important. In Mace, it is part of the mark scheme; in BP it isn't, but a judge can't help favouring a speech that is delivered in a way that is engaging and easy to listen to.
Here are some basic principles for how you should deliver your speeches.
1. Speak loud enough. Speeches that can't be heard are not likely to be persuasive.
2. Speak clearly. Mumbled, unclear speeches are also not very persuasive. However, provided you are clear, you needn't be afraid of speaking fast. Normally, when advising people about giving readings or speeches in public, I recommend speaking slowly, to allow the audience to take in what you are saying. Debating is different, however; you have a lot of content to get in, and you need to make the most of every second of your limited time.
3. But don't shout. No one likes being shouted at. Aim to speak to the furthest away person in the room. If you have difficulty with projecting your voice, ask for help from a drama teacher.
4. Make eye contact. It makes the people who are listening feel like you are talking to them. If you never look up from your notes, they will feel like you don't care about them. Choose a different person to make eye contact with for each section of your speech. Don't forget the judges. However, don't make continuous eye contact with only one person throughout your speech. They might think you are either threatening them or trying to pick them up.
5. Vary your tone and your pace. Talking in the same way throughout your speech is boring.
6. Use emotion. Sound like you care about what you are arguing for, or against.
7. Use timing. A slowing down of pace, or a dramatic pause, just before a major point, will give it more impact.
Film yourself speaking and see how well you do all these things and / or ask someone else to comment on your performance.