It's easy to speak very rapidly when you're nervous, but make a conscious effort to slow down. Then slow down some more. Make sure everyone can hear you. A conversational voice-level will be inaudible to many of the students. Ask the students in the back of the room if they can hear you well enough.
Eye contact is vital.
Look at your students when you speak. This sounds like obvious advice, but too many of us forget to follow it. Of course you shouldn't stare down your class, but eye contact will help to maintain students' interest, as well as let you know when attention may be wandering. Vary your tone of voice and intonation. No one likes to listen to a monotone.