When you are making a presentation, your posture can affect the quality of your voice. The way you stand, you know, the position of your head, neck and shoulders and feet and so on is important. Check that your feet are parallel and apart and your weight is slightly forward on your feet. Make sure that your knees are relaxed; as if you are tense your back will suffer.
Pay special attention to your head. Look straight ahead and do not push your neck out or drop it on to your chest.
Now most of us feel quite nervous before we speak in public. You will feel better if you spend some time dealing with the tension. It is a good idea to try and think about what it is that is making you feel this way. That way you can have some control over it. Next, try and locate the area of tension in your body. Often it is your neck or your shoulders. Then concentrate on massaging these parts and consciously trying to relax them. Believe me, it works.
Let us think about what you say, now. To maximize your performance make sure you are well prepared. Look over your notes, practice what you want to say, preferably out loud, and then, perhaps most important of all, try to feel you really want to share your subject with your audience. If you feel and share your enthusiasm with them, you are more than half-way there.
Remember that how people feel about you and what you are saying to them will depend on your body language. There are three main behavior types: passive, aggressive and assertive. You can use any of these types, although I think the assertive posture is one that suites most occasions best.
The passive body type has a withdrawn posture. You may fidget a bit with your hands and hair. In fact, I remember a well-known politician who whenever he was speaking would constantly message the top of his head. So beware of those funny little mannerisms. They can become intensely irritating to an audience.