I finally found the secret! Stretch interfacing! Up until a few months ago, I had no idea that there was such a thing. Basically, the interfacing just adds thickness and stability to your knit fabric-- without hampering it's stretchabilty.
I was able to work with some of Pellons Xtreme Stretch interfacing and am elated for with the results! This opens up tons of possiblities for me-- pj's, yoga skirts and more!
You can check out my Yoga Skirt tutorial on Pellon's blog, or download a printable version of the tutorial on the Pellon Projects website.