Leslie at Marveles Art Studio,
http://marvelesartstudios.blogspot.com/ is a Bernina Educator and has done several posts on free motion couching. They're older posts so you may have to dig a little to find them!
I am jealous, the Bernina FM couching foot seems to be well designed. The one Janome came out with has been a disappointment.
When I do FM couching though, I lay the yarn real loose, pulling several yards out, on my sewing coutertop space as the less tension on the yarn, the better.