Hi grumpy,
You have a tough, though not impossible, photo taking challenge. You're right to and should be commended for practicing so much in advance.
Using the fastest shutter speed possible is going in the right direction, but that also lets in less light so the aperture opens decreasing depth-of-field (the range a subject is in proper focus).
From an artistic standpoint, not all blur in a photo is bad when it captures some of the movement which is taking place on the stage. The trick is to prevent the entire image from being blurred, and for that you will have to do everything to prevent camera shake.
We have some suggestions at the main site about taking photos at school plays. I'm sure some of the info there will help.
http://www.digicamhelp.com/learn/camera-questions-and-answers/taking-photos-at-a-school-play.htmIf you have futher questions, let us know.