Believe in the message you are communicating. If this is not possible -- for example, because someone assigned you the topic for a speech or you're participating in a debate -- rehearse until you can convince others that you truly believe what you're saying.
Use open body language and hand gestures when speaking. Try not to cross your arms, fidget or look down as these movements will make you appear less confident and approachable.
Look people in the eye whenever you are speaking to them. If you frequently dart your eyes away from your listeners, they may take that as a sign that you are insincere.
Project your voice, enunciate your words and use declarative statements when speaking. People will trust you more readily if you speak with confidence.
Confirm that your audience has received the message you intended to convey. Provide more information, if necessary, to clarify your points.