One of the first modern Master-Apprentice Programs (also known as Mentor-Apprentice) was started in California through the Native California Network, in 1993. These programs are especially useful when speakers of a language are elderly, and so can concentrate on transmitting their knowledge to a 1-3 learners who are dedicated to learning and teaching this language.
Resources for beginning a Master-Apprentice Program
How to Keep Your Language Alive by Leanne Hinton, Matt Vera, and Nancy Steele
BC Master-Apprentice Handbook by First People's Cultural Council