There's a good instructional tape from Texas music and video(1-800-874-8384), "How To Yodel" by Tania Moody and Rusty Hudelson. They teach you to find the narural "break" in your voice. This worked OK for me, but interestingly, my wife, who is a "trained" singer and therefore has spent years learning how to not have your voice break, has a VERY difficult time yodeling. And she's the greatest singer in the world in my opinion(of course, I might be biased).