I just gave a speech about public speaking, using my experience as an actor, teacher, and writer.  

The main points:

• You have to be yourself, and you have to communicate with the audience.
• You have to practice, and practice well.

I was not surprised to discover that the best books on the subject were by ex-actors.  Speak Without Fear, by Ivy Naistadt, offers great tips and exercises for all those who find public speaking to be a fate worse than death.

Taking Center Stage: Masterful Public Speaking, by Deb Gottesman and Buzz Mauro, uses all the skills you learn in acting class to help public speakers.

All public speaking advice begins and ends with the word practice.  But I would add that it must be good, useful practice, not just going through it wrong over and over again. You need a plan.  And you need an audience.

Actors call it rehearsal.  Actors rehearse with a director.
 
Athletes call it practice.  Athletes practice with a coach
. 
The director or coach creates a practice plan; then tells the performer when they’re on, and off track.  A good director stands in for the audience.  A good coach leads the athlete to do her best.

Don't practice hard.  Practice well!