No matter what you are presenting, keep in mind that you are the viewer's connection with what is about to occur on screen. Like a host at a party, your job to to make people feel comfortable and familiar with something or someone that is essentially new or unknown to them. Your job is not to be the center of attention. Ostentatious, attention-grabbing actions are not appropriate. Instead, think of yourself as the grease that makes the cookies not stick to the pan.
In some cases, you will ad-lib what you say, but most of the time, you will work from prepared remarks. The manner in which you receive these remarks varies, but the main methods are via teleprompter, cue cards, or straight memory. Regardless of the method, it is essential that you make eye contact with the camera, as if it were a person to whom you were speaking. Some people get nervous looking into the lens of the camera. Therefore, focus on looking past the lens. It will make you seem more natural on screen and can help with nerves.
Another way to combat nerves and give a smoother performance is to pretend you are playing a role. Do not think of yourself as the person speaking to the camera. Instead, play the part of the charming host, the comedic host, the serious presenter, the energetic salesman and so forth. When you think of yourself in this manner, the appropriate vocal inflections will come more easily for the occasion. Find the right attitude for the subject matter, and stick to it.