I haven't yet tried it myself, but if you look at the youtube comments of the following videos, they seem to be effective:
Male voiced: http://www.youtube.com/watch?v=4dEC5K8xYPc
Femaled voiced: http://www.youtube.com/watch?v=XunrdxreD4M
Blog post this comes from: http://hypnofantastico.blogspot.com/201 ... slave.html