Great question!
Ask yourself one thing - when you hear a good public speaker, what is the one thing they do or say that makes you want to listen to that person?!
Passion could be one answer, knowledge, willingness to share... as soon as one finds that sparkle, you then become animated, motivated and people want to hear what you have to say.
It takes a great deal more to be proficient, but when you have that intrinsic desire to share, the rest often follows naturally!
Don't forget practice, good posture, eye contact, positive nonverbal communication, (among a few) these all are part of what makes a public speaker - a GREAT speaker!
Speak of experience, from the heart and relate to the audience! :)
