Greg Low, Australian RD, has recorded a four-part webcast on Speaking at Large Events such as TechEd. These are full of good advice.
My advice to those who want to speak is pretty simple: start speaking. Your user group, code camps, heck start with your dog if you can't get invited anywhere. In fact, start with your dog for rehearsals even if you do get invited somewhere. Just hearing yourself get all tangled up and lost 5 minutes into the talk will impress upon you the need to have an outline and a plan, to rehearse, and not to try to memorize every sentence. Every time you give a talk you will get better, and every time you hear one you will get better, so go to things. A lot of things.
Greg covers some nice practical details that I won't repeat - watch his videos!