It's natural to be nervous when giving a presentation and members of your audience understand that. Just because you're nervous on the inside, though, doesn't necessarily mean you're showing it on the outside. Most of the time your nervousness is not as bad as you think it is, so if you don't say anything, nobody will notice. However, if you apologize, you'll only succeed in calling attention to the fact that you're nervous, and worse yet, apologizing won't help eliminate it. In fact, it may make it worse because-- congratulations!-- now everybody knows you're really nervous! The bottom line is that you should never apologize for being nervous because had you remained silent, your listeners may have never thought that you were.
Keep it simple. The aim is to communicate, not to show off your vocabulary. Using complex words or phrases increases the chance of stumbling over a word or losing your train of thought.
Emphasize the key points. Make sure people realize which are the key points of your study. Repeat them using different phrasing to help your audience remember them.
Check the pronunciation of difficult, unusual, or foreign words beforehand. Keep it simple, but if you have to use unfamiliar words, write them out phonetically in your notes. This is particularly important when pronouncing proper names.
Speak loudly enough for everyone in the room to hear you. This may feel uncomfortably loud at first, but if people can't hear you, they won't try to listen.
Speak slowly and clearly. Don’t rush! Speaking fast makes it harder for people to understand you.
Practice to avoid saying um, ah, you know, like. These words occur most often during transitions from one idea to another and are distracting to an audience. The better you know your presentation, the better you can control these verbal tics.
Vary your voice quality. If you always use the same volume and pitch [for example, all loud, or all soft, or in a monotone] your audience will stop listening.
Speakers with accents need to slow down [so do most others]. Non-native speakers often speak English faster than we slow-mouthed native speakers, usually because most non-English languages flow more quickly than English. Slowing down helps the audience to comprehend your talk.
When you begin a new point, use a higher pitch and volume. Use the tone of voice to help emphasize the transition to a new point.
Slow down for key points. These are also moments in your presentation to consider using body language, such as hand gestures, to help emphasize key points.
Use pauses. Don't be afraid of short periods of silence. They give you a chance to gather your thoughts, and your audience a chance to think about what you've said.