Practice
There's a good reason that we say, "Practice makes perfect!" You simply cannot be a confident, compelling speaker without practice.
To get practice, seek opportunities to speak in front of others. For example, Toastmasters is a club geared specifically towards aspiring speakers, and you can get plenty of practice at Toastmasters sessions. You could also put yourself in situations that require public speaking, such as by cross-training a group from another department, or by volunteering to speak at team meetings.
If you're going to be delivering a presentation or prepared speech, create it as early as possible. The earlier you put it together, the more time you'll have to practice.
Practice it plenty of times alone, using the resources you'll rely on at the event, and, as you practice, tweak your words until they flow smoothly and easily.
Then, if appropriate, do a dummy run in front of a small audience: this will help you calm your jitters and make you feel more comfortable with the material. Your audience can also give you useful feedback , both on your material and on your performance.
Engage With Your Audience
When you speak, try to engage your audience. This makes you feel less isolated as a speaker and keeps everyone involved with your message. If appropriate, ask leading questions targeted to individuals or groups, and encourage people to participate and ask questions.
Keep in mind that some words reduce your power as a speaker. For instance, think about how these sentences sound: "I just want to add that I think we can meet these goals" or "I just think this plan is a good one." The words "just" and "I think" limit your authority and conviction. Don't use them.
A similar word is "actually," as in, "Actually, I'd like to add that we were under budget last quarter." When you use "actually," it conveys a sense of submissiveness or even surprise. Instead, say what things are. "We were under budget last quarter" is clear and direct.
Also, pay attention to how you're speaking. If you're nervous, you might talk quickly. This increases the chances that you'll trip over your words, or say something you don't mean. Force yourself to slow down by breathing deeply. Don't be afraid to gather your thoughts; pauses are an important part of conversation, and they make you sound confident, natural, and authentic.
Finally, avoid reading word-for-word from your notes. Instead, make a list of important points on cue cards, or, as you get better at public speaking, try to memorize what you're going to say – you can still refer back to your cue cards when you need them.
Pay Attention to Body Language
If you're unaware of it, your body language will give your audience constant, subtle clues about your inner state. If you're nervous, or if you don't believe in what you're saying, the audience can soon know.
Pay attention to your body language: stand up straight, take deep breaths, look people in the eye, and smile. Don't lean on one leg or use gestures that feel unnatural.
Many people prefer to speak behind a podium when giving presentations. While podiums can be useful for holding notes, they put a barrier between you and the audience. They can also become a "crutch," giving you a hiding place from the dozens or hundreds of eyes that are on you.
Instead of standing behind a podium, walk around and use gestures to engage the audience. This movement and energy will also come through in your voice, making it more active and passionate.