Patsy Rodenburg has written a very good book called "The actor speaks". I have read the book and I can recommend it to the voice training for all public speakers. There is also DVD existing about her Voice Classes.
Here you have a link for Nationa Theatre Vocal Warm Up Tips. It has many same methods as you can find from Patsy Rodenburg's book and DVD.
Patsy Rodenburg Web (click name)
Michael Caine Acting in Film Workshop - (Part 1) You can find the rest of the links on YouTube (Part. 2-6)
Valentine Palmer an Actor and Voice Coach. He has many great videos for voice improvement.
If you are interested in film acting there is a free e-book The Tao of Acting on Dr Plonkey's homepages. Good introduction for acting especially self-learners.
(A Book about marketing, networking and visualization to achive your Acting success)
You will find the rest of the links on YouTube (Part. 2-6
Sanford Meisner technique is widely recognized and used. Meisner held acting classes, yet at the age of 92 until his death. I've gone through this 8-hour course and can recommend it to everybody who wants to learn his method. Sanford has a weak voice due to troath surgery, but he teaches this course with great passion.