Practice constantly talking out loud about your subject. Growing up my dad was in medical school, and I would hear him at night reciting/practicing, and would think he was pretty silly. He kept a better than 4.0 through 10 years of med school. It worked wonders for myself later when I had to give instruction in the Army, just take about 20 minutes and practice talking out loud as if you were not having a conversation, but actually teaching the subject. It may seem silly, but most people have never done it...usually when you talk everyday people are having a conversation and giving you feedback.
If you want to really see this method in action, watch drill sergeant trainees yelling at trees for hours on end.
Public speaking is like any other motor-memory skill, the more you practice, the more comfortable and natural you'll be when it comes "game time." When you practice, you'll see its not much different whether the audience is actual people or just the crap sitting around your bedroom.