Whenever you’re giving an important speech (such as a keynote) it’s important that you practice. Practicing out loud in front of a mirror is good. Practicing in front of a test group (such as a Toastmasters club) will help you get some feedback.
If you really want to improve your speech quickly, record it. Audio only is fine because it’ll force you to improve the speech’s content. You can also listen to it in the car, at work, while you jog, etc… so you’ll have the content of your speech perfected and memorized. If you have the ability to edit audio and create CDs, that’s even better as you can sell the recording at the event.
If you have access to a camcorder, go ahead and use it in addition to the audio (you’ll still want the audio-only piece for listening in the car, work, etc…). You’ll be able to see yourself and evaluate your body language, gestures, and movements. You’ll also be able to fix any behavior that might prove to be distracting to the audience such as pacing, repetitive gestures or other movements due to nervousness. Again, if you have the ability to edit the video and can record yourself practicing in front of a live audience, you can sell that as a product at the event as well.