Apparently public speaking is one of the most common phobias in the world.
For me, I would say, forget all the advice about imagining people in their underwear. Instead, use that imagination to picture yourself succeeding. Try pretending to be someone else, someone confident and outspoken. Not necessarily a real individual, but perhaps the imaginary epitome of what you want to be like when you speak. Perhaps a town crier, shouting proudly. Or a suavely dressed suit giving a presentation.
You could even include a tiny prop to help you build this image. eg. get a shirt that needs cufflinks and buy yourself a pair of stylish cufflinks. As you get dressed for your speech, and put the cufflinks in, imagine that with those cufflinks, you leave behind your self and take on the persona of the confident public speaker.
Another hint: have a glass of water with you. That way, if you have one of those brain-freeze moments, you can take a sip of water; the pause will look natural and give you a chance to find your thoughts (works for interviews too)
Also, try walking up to the podium/stage etc. with big strides. Your body memory is set that when you walk with big strides you are being confident. Because of that, if you deliberately walk with big strides, your body will tell your brain that you are feeling confident, and it will calm you down. Seriously, this works!
I hope this advice is of help.
My other small advice, simply put: KNOW YOUR SUBJECT.
good luck everyone!